Viscosity-modified lactide polymer composition and process for manufacture thereof

ABSTRACT

A composition comprising a polylactide polymer with improved extensional viscosity and methods of making the same are disclosed. The polylactide polymer composition is prepared by providing in the composition polylactide polymer molecules which have been modified, relative to linear non-substituted polylactide, to provide increased molecular interaction among polylactide backbone chains in the composition. The preferred polylactide polymer composition has a number average molecular weight of at least about 10,000 (preferably at least 50,000) and a polydispersity of at least about 2.5. In addition, the polylactide polymer composition should have a neck-in ratio of less than about 0.8.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present is a continuation-in-part of U.S. Pat. No. 5,359,026, U.S.Ser. No. 08/100,550, which was filed on Jul. 30, 1993. The disclosure ofU.S. Ser. No. 08/100,550 is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to selected lactide polymer compositionsand processes for manufacturing such compositions.

BACKGROUND OF THE INVENTION

The present disclosure concerns ongoing efforts in developing lactidepolymers useable in preferred manners. U.S. Pat. No. 5,142,023 issued toGruber et al. on Aug. 25, 1992, the disclosure of which is herebyincorporated by reference, discloses, generally, a continuous processfor the manufacture of lactide polymers from lactic acid. Selectedpolymers according to U.S. Pat. No. 5,142,023 have physical propertiessuitable for replacing petrochemical-based polymers for packaging,paper-coating and other applications. Related processes for generatingpurified lactide and creating polymers therefrom are disclosed in U.S.Pat. Nos. 5,247,058, 5,247,059 and 5,274,073 issued to Gruber et al.,the disclosures of which are hereby incorporated by reference.

Generally, commercial exploitation of polymers utilizing processes suchas those disclosed in the patents to Gruber et al. can involveconversion of raw material monomers into polymer beads, resins, or otherpelletized or powdered products. The polymer in this form would then besold to end users who would extrude, blow-mold, cast films, blow films,foam, thermoform, injection-mold, fiber-spin or otherwise convert thepolymer at elevated temperatures, to form useful articles. The aboveprocesses (and related processes) are collectively referred to herein as"melt-processing". Polymers produced by processes such as thosedisclosed in the patents to Gruber et al., and which are to be soldcommercially as beads, resins, powders or other non-finished solidforms, are herein generally referred to collectively as polymer resins.These polymer resins, if biodegradable, can help alleviate theenvironmental stress due to disposal of items such as packagingmaterials, coated paper products, films, single use diapers and thelike.

It is generally known that lactide polymers or polylactides areunstable. The concept of instability has both negative and positiveaspects. A positive aspect is the relatively rapid biodegradation orother degradation that occurs when lactide polymers or articlesmanufactured from lactide polymers are discarded or composted aftercompleting their useful life. A negative aspect of such instability isthe potential for degradation of lactide polymers during processing atelevated temperatures, for example during melt-processing by end-userpurchasers of polymer resins. Thus, some of the same properties thatmake lactide polymers desirable as replacements for relativelynon-degradable petrochemical polymers also can create undesirableeffects during production of lactide polymer resins and processing ofthose resins.

Lactide polymers are subject to unwanted degradation during meltprocessing via a number of pathways. These pathways include hydrolysisand other side reactions, which, for example, result in lactideformation and decreased molecular weight of the polymer. Furthermore, asprocessing temperatures are increased (especially to above about 230°C.), lactide polymer degradation is substantially and undesirablyaccelerated. Accordingly, even if a relatively melt-stable lactidepolymer can be produced, it would be generally desirable to provide alactide polymer or resin formulation that can be processed into usefularticles at reduced temperatures (i.e., especially and preferably at nomore than about 180° C.).

During certain melt processing operations, linear polymers such aslinear polylactide exhibit certain undesired flow properties, such asnecking. For example, if polylactide is extruded as a film onto a movingsubstrate, the film of polylactide being directed onto the substratewill tend to neck under the tensional forces caused by the movingsubstrate. By "necking" in this context it is meant that the width ofthe film will tend to narrow as the film is pulled or stretched. Thisleads to problems with control of the process and problems withmaintaining consistency in film thickness, etc. Specifically, incomparison to polypropylene or polyethylene, linear polylactides (PLA)typically exhibit substantially more problem necking and less meltstrength. Linear polymers, such as PLA, also tend to exhibithydrodynamic instability or draw resonance at high draw ratios. Thisdraw resonance can cause a periodic variation in a coating width and/orgauge, for example, and can lead to rupture of the polymer web.

Moreover, in a coating application or blown film production the polymermust withstand various forces such as acceleration in going from the dieto the substrate in a coating application or the gas pressure thatcauses stretching in a blown film. The ability to withstand these forcesis referred to as "melt-strength". There has been a need for lactidepolymer formulations that will have improved melt-strength.

SUMMARY OF THE INVENTION

Polylactide polymer compositions with improved melt-strength andrheology and methods for making the same are disclosed. The methodsinclude providing in the polylactide polymer composition, polylactidepolymer molecules which have been modified, relative to linearnon-substituted polylactide, to provide increased molecular interactionamong polylactide backbone chains in the composition. The polymercomposition can (and preferably will) have at least one of thefollowing, relative to linear non-substituted polylactide: an increasedweight average molecular weight, increased branching and/or increasedbridging. Preferably, the polymer has a number average molecular weightfrom about 10,000 (and more preferably at least 50,000) to about300,000.

In addition, the preferred polymer compositions preferably have aresidual monomer concentration of zero to about 2 percent by weight; anda water concentration of zero to about 2000 parts per million. Thepolymer should preferably have a weight average molecular weight fromabout 100,000 to about 1,200,000.

In many useful and preferred applications, the method will involveproviding modified polylactide polymer molecules having sufficientmolecular interaction to produce a polymer composition having apolydispersity of at least about 2.5. One manner in which this molecularinteraction can be provided is generating bridging between polylactidemolecules through free radical reaction. Such bridging can, for example,be generated by using a molar ratio of free radical initiator to polymerwithin a range of 0.01:1 to 10:1.

Preferably, sufficient molecular interaction is provided such that apolymer composition having a measured natural log of the intrinsicviscosity (in deciliters per gram) of at least 0.1 below a measurednatural log of the intrinsic viscosity (in deciliters per gram) of alinear unsubstituted or non-substituted polylactide of comparableapparent weight average molecular weight (as measured by gel permeationchromatography) is produced. In addition, preferably sufficientmolecular interaction is provided such that a polymer composition havingreduced neck-in when processed, relative to a linear non-substitutedpolylactide of comparable weight average molecular weight, is produced.The neck-in should (and may) preferably be reduced such that a neck-inratio for said polymer composition is less than about 0.8.

The method of producing the polymer may preferably involve formingpolylactide molecules in a procedure including a reactant in addition tounsubstituted lactic acid or lactide. Preferably, the reactant providedincludes: a non-initiating lactide reactant, an initiating reactant, acombination reactant and/or mixtures thereof. The reactant other thanlactic acid or lactide can be an initiating reactant having oneinitiating group therein. The initiating group can be either an hydroxylgroup or an amine group. Such a reactant would preferably contain abulky organic group therein.

The reactant other than unsubstituted lactic acid or lactide can havemore than one initiating group therein. These initiating groups can behydroxy groups, amine groups, and/or mixtures thereof.

The reactant other than unsubstituted lactic acid or lactide can be anon-initiating lactide reactant containing one or more non-initiatinggroups selected from: epoxides; cyclic esters; and, combinationsthereof. Also, combination reactants (including both initiating andnon-initiating groups) may be used. In some applications, the reactantin addition to unsubstituted lactic acid or lactide can be anon-initiating lactide reactant that contains at least one carbon-carbondouble bond. In still other applications, the reactant other than lacticacid or lactide can contain a bulky organic polymer entangling grouptherein.

Certain applications of the invention are directed toward compositionscomprising: a polylactide based polymer composition having a numberaverage molecular weight of at least 10,000 (and preferably at least50,000); and preferably a polydispersity of at least 2.5. Preferably,the polymer has a weight average molecular weight of at least about100,000 and not greater than about 1,200,000. Preferably, thepolylactide-based polymer composition is provided to have a neck-inratio of less than about 0.8. In addition, the polymer composition canhave sufficient molecular interaction such that its intrinsic viscosityis at least 0.1 deciliter per gram below an intrinsic viscosity of alinear, non-substituted polylactide of comparable apparent weightaverage molecular weight, as measured by gel permeation chromatography.

The invention also includes a composition comprising the result of: (a)providing lactide or polylactide polymer; (b) providing a reactant otherthan unsubstituted lactic acid or lactide; and (c) reacting the lactideor polylactide polymer with the reactant to obtain an improvedpolylactide polymer composition. This polylactide polymer compositionshould be produced to have increased molecular interaction amongpolylactide backbone chains relative to a linear, non-substitutedpolylactide of comparable weight average molecular weight; a numberaverage molecular weight of at least 10,000 (and preferably at least50,000); and also preferably a polydispersity of at least 2.5.

It is an advantage to the present invention that improved polylactidepolymer compositions can be made from a lactide mixture which has notbeen recrystallized from a solvent. That is, the lactide mixture mayinclude initiators such as small amounts of water or lactic acidtherein, yet improved polymer compositions according to the presentinvention (for example, those having a number average of molecularweight of at least 50,000) will still result. Preferred methodsdisclosed herein for accomplishing this involves reacting the lactidemixture which has not been recrystallized from a solvent (or a polymerresulting from a lactide mixture which has not been recrystallized froma solvent) with a non-initiating lactide reacting containing at leasttwo non-initiating groups each selected from: epoxide groups; cyclicester groups; and, combinations thereof. An alternate method useable toaccomplish the desired result, disclosed herein, is using radicalreactions to generate linking, or the introduction of a cross-linkablegroup into the polymer molecules. Also, chain extenders can be used.Variations of these approaches, and others, will be apparent from thedetailed description below.

It is still a further advantage to the present invention that it may beapplied in a continuous process production of polylactide-basedpolymers. That is, the various reactants can be inserted into acontinuous process, with a sufficient control, to yield the desiredpolymer product. The reactants can be introduced as the lactide iscontinuously fed into a polymer reactor, for example, or downstreamtherefrom. Variations of this approach will be apparent from thefollowing detailed descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred process for themanufacture of a melt-stable lactide polymer.

FIG. 2 is a graph of the natural log of a linear lactide polymer'sintrinsic viscosity with respect to the natural log of the polymer'smolecular weight.

FIG. 3 is a graph of the apparent shear viscosity of three PLA polymerswith respect to the apparent shear rate at a temperature of 175 degreesCelsius.

FIG. 4 is a graph of the apparent shear viscosity of two PLA polymerswith respect to the apparent shear rate at 175 degrees Celsius.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concerns methods of improving polylactide polymerswith respect to rheology (melt flow) and melt strength characteristics.In particular the invention concerns improvements in the rheology and/ormelt strength of the molten polymer which tend to lessen propensities to"neck" or exhibit similar phenomena. The invention preferably concernsaccomplishment of such improvements without undesirably affecting otherpreferred characteristics of preferred polylactide polymers including,for example: compostability and/or biodegradability characteristics;melt stable characteristics; and the characteristic of being able to beraised sufficiently above t_(g) (glass transition temperature orsoftening point) for accomplishment of a fluid material of appropriateflow characteristics for processing, without reaching temperatures atwhich substantial or undesirable levels of degradation begins to occur.

The improved processing features achievable in some applications of thepresent invention include lower temperature processing, lower powerconsumption and pressure, and increased melt strength and improved meltflow characteristics. The polymers of the present invention may be meltprocessed into films, sheets, coatings for paper, blow molded articles,fibers, foam, foamed articles, thermoformed articles, injection moldedarticles, non-woven fabrics and the like. These articles may thereafterbe components of various commercial articles, such as films for diapers.

Rheology

In general, the rheology characteristics of a resin or polymer are itsviscosity or flow characteristics. For polymers such as polylactide(PLA), i.e. thermoplastic polymers, rheology or flow characteristics areused in reference to the characteristics exhibited by the polymer oncethe temperature of the polymer is raised above t_(g) (or melting pointif a crystalline polymer is involved). Generally, the concern is withrespect to the flow characteristics of the polymer once it has beenraised to a sufficient temperature that viscosity is reduced to a pointwhere various melt processing steps are feasible.

Typically, for polylactide polymers (PLA) melt processing is feasiblewhen the shear viscosity of the resin has been reduced to at least about10,000 Pa-s (Pascal-seconds), and typically to within a range of about 1Pa-s to about 1,000 Pa-s. For typical polylactide polymers such as thosedescribed in U.S. Pat. No. 5,142,023 to Gruber et al., t_(g) is about50° C. to about 65° C., and the materials are typically heated to about160° C. to about 200° C. for processing.

With respect to rheology of linear polymers, various characterizationsare typically made with respect to viscosity. Typically the term"viscosity" is used to characterize the melt flow characteristics of (orthe flowability of) the polymer. With respect to these melt flowcharacteristics, two types of viscosity are generally consideredimportant. One of these is shear viscosity, which generally relates toevaluations of capillary flow, i.e. how much of the molten polymer canflow through a capillary tube within a given period of time, etc. Forexample, in the paper coating industry, shear viscosity is used toindicate the force which will be needed to push the polymer through anextruder die. A higher shear viscosity indicates that a larger force isrequired to push the polymer resin through processing equipment, such asan extruder die, and a lower shear viscosity indicates that a lowerforce is required to push the polymer through processing equipment.

The other type of viscosity characteristic which is important is relatedto extensional viscosity. Extensional viscosity refers to viscosity inthe absence of shear, and generally relates to the resistance of thepolymer to flow when pulled or drawn. A higher extensional viscosityindicates that the resin is very resistant to flow when pulled or drawn,and a lower extensional viscosity indicates that the resin is not veryresistant to flow when pulled or drawn.

Extensional viscosity is particularly important with respect to meltprocessing and the characteristic of necking. Change in extensionalviscosity at increasing strain rate, and the time-dependent response ofthe polymer in extensional flow, can also be important with respect tomelt processing operations. Collectively these define the extensionalviscosity characteristics. A difficulty with conventional polylactidesis that they are prone to neck, because of poor extensional viscositycharacteristics.

Development of desireable polymers for melt processing requires, inpart, development of a desireable balance of extensional viscositycharacteristics and shear viscosity. If the extensional viscosity is notonly increased, but the shear viscosity is also increased substantially,the characteristics of the polymer melt may be affected such that it isno longer desirable for melt processing. For example, if both theextensional and shear viscosities are substantially increased byincreasing molecular weight, a lactide polymer resin may not flowsufficiently readily through conventional processing equipment (atconventional processing temperatures) to be widely acceptable. If thetemperature in the processing equipment is increased to compensate forthis lack of flowability, undesirable degradation of the polymer may beaccelerated during polymer production or melt processing. Also, forexample, if both extensional and shear viscosities are substantiallydecreased by decreasing molecular weight, a lactide polymer resin mayrequire less force to flow readily through the conventional processingequipment, but the resin will be more prone to neck.

Further, a substantial advantage to the use of polylactide polymer inthe formation of products is that, in general, polylactide isbiodegradable or compostable. If the polymer is modified in such a waythat the extensional viscosity characteristics are substantiallyincreased, but significant losses in compostability or the ability tobiodegrade the polymer occur, the tradeoffs may be unacceptable for wideutilization or acceptance of the material.

In general, for linear non-substituted PLAs it can be shown that as themolecular weight of the polymer increases, a plot of intrinsic viscosityversus apparent molecular weight, as measured by gel permeationchromatography (GPC) follows a well-defined curve. In addition, a highermolecular weight (i.e., above a critical molecular weight) lactidepolymer is preferred, because the physical properties such as modulus,tensile strength, percentage elongation at break, impact strength,flexural modulus, and flexural strength remain relatively constant whenthe lactide polymer is above a threshold molecular weight. The lowerlimit of molecular weight of the preferred polymer compositions of thepresent invention is preferably set at a point above this threshold inorder to result in a lactide polymer with more predictable physicalproperties upon melt-processing. In general, this critical "lower"number average molecular weight is at least about 10,000 (and preferablyat least 50,000), and a preferable "lower" weight average molecularweight is at least about 100,000.

The practical upper limit of the molecular weight is based upon apractical upper limit of workable viscosity (viscosity generallyincreases with increased molecular weight). In order to melt-process avery high molecular weight lactide polymer, the melt-processingtemperature must be increased to reduce the viscosity of the polymer. Asthe processing temperature is increased, however, undesirabledegradation of the lactide polymer is accelerated.

The exact upper limit on molecular weight may vary depending on theparticular melt-processing application since required viscosities vary,and residence time within the melt-processing equipment also varies.Thus, the degree of degradation, for a given polymer, in each type ofprocessing system will also be expected to vary. It is believed that onecould readily determine the suitable molecular weight upper limit formeeting the viscosity and degradation requirements in any selectedapplication, however. Generally, the number average molecular weight ofthe preferred polymer will not be greater than about 300,000 and theweight average molecular weight will not be greater than about1,200,000.

The Linear Nature of PLA

In general, poor extensional viscosity characteristics and rheologycharacteristics which lead to significant amounts of necking, arecharacteristics of linear polymers, and PLA is a linear polymer.Approaches to improving PLA, according to the present invention,generally concern methods of increasing interaction between the longpolymer chains of individual molecules sufficiently to improve rheology,while at the same time not introducing so much interaction that othercharacteristics such as compostability, biodegradability, andmelt-processability, are undesirably affected. Thus, an improved ormodified polylactide polymer, in accord with the present invention, isone in which the interaction between the long polymer chains ofindividual molecules is increased such that rheology is improved,without substantially undesirably affecting compostability,biodegradability, and melt-processability of the polymer. In general,the methods described herein concern modifications that can be madeduring polymer resin formation. Thus, initially, a brief considerationof the formation of linear PLA is presented.

In general, linear (unsubstituted) PLA is formed from ring openingpolymerization of the cyclic dimeric ester of lactic acid, i.e. lactide.This is described, for example, in U.S. Pat. No. 5,247,059 to Gruber etal. While the precise nature of the polymerization is not fullyunderstood, in general it appears to concern chain propagation in thefollowing manner. An initiator having a group containing an activemoiety (such as a --OH group) therein is provided and mixed with thelactide. The initiator may comprise, for example, water, an alcohol,lactic acid, amines or other materials. The "active moiety group" reactswith one of the carbonyl groups of the cyclic dimer, to open the ring.Each ring opening results in the generation of an active --OH group onthe end of the polymer backbone. The newly generated active --OH groupcan react with another lactide molecule, to ring open. Chain propagationthus occurs in a linear fashion. The length of the chains, i.e. themolecular weight of the resulting polymer, will in part depend upon thenumber of active --OH groups initially provided; and the rate ofreaction and length of time allowed. If each initiator has only one ortwo, active --OH group(s) thereon, in general, the resultant polymerwill be a linear polymer with one or two hydroxyl terminated ends. Ingeneral, as more equivalents of initiator are provided, the molecularweight of the resulting polymer will be lower. That is, in general,molecular weight is inversely proportional to the number of initiators.

Approaches to generating Interaction Between Long Polymer Chains

As indicated above, generally, improving extensional viscositycharacteristics in a linear polymer can be accomplished by providinginteraction between the long polymer backbones. Providing interactionbetween the long polymer backbones, typically, can be accomplished byincreasing the weight average molecular weight of the lactide polymermelt, providing branching within the lactide polymer, and/or providingbridging in the lactide polymer. In this context, "bridging" refers tobonding between long polymer PLA-based chains. The term "branching"refers to either providing pendent groups from a linear PLA-basedpolymer chains or providing long polymer segments joined to one anotherthrough a residue. The term "PLA-based polymer chains" refers to polymerchains in which the majority of repeat polymer units or residues areunsubstituted lactic acid or lactide residues. Preferably they compriseat least 50% by weight residues from lactic acid or lactide. Providingbranching and/or bridging in the lactide polymer can lead to a lesslinear polymer.

Increasing Weight Average Molecular Weight

Increasing the weight average molecular weight of the polymer is a meansof increasing interaction between backbone chains of the polymer becausethe higher the molecular weight, the more likely the polymer chains willinteract with one another via molecular entanglement. The weight averagemolecular weight is the summation of the product of the molecular weightof the species, squared, and the number of molecules of the species,divided by the summation of the product of the molecular weight of thespecies and the number of molecules of the species.

The number average molecular weight of a polymer is the weight of agiven sample of polymer divided by the number of molecules within thatsample. For example, if a polymer mixture includes one polymer moleculeof molecular weight 100,000 and two polymer molecules of molecularweight of 10,000 each, the number average molecular weight for thecomposition is 40,000, whereas the weight average molecular weight is85,000.

Polydispersity is one indicator of increased weight average molecularweight and thus one indicator of increased interaction between the longpolylactide polymer backbones. In general, the polydispersity (orpolydispersity index or polydispersion index) of a polymer is defined bythe relationship between the weight average molecular weight of thepolymer and the number average molecular weight of the polymer.Specifically, polydispersity index is the ratio between weight averagemolecular weight and number average molecular weight. Therefore, anincrease in polydispersity index can indicate an increase in the weightaverage molecular weight of the polymer, if the number average molecularweight of the polymer is held substantially constant.

The polydispersity index of linear polylactides prepared according tothe methods of Gruber et al. as disclosed in U.S. Pat. Nos. 5,247,059and 5,274,073 are generally within the range of about 1.5 to 2.5 and aretypically about 2. Generally, the polydispersity indices of preferredpolymers in accord with the present invention are at least about 2.5 andmore preferably at least about 3. The higher the polydispersity index,in general, at substantially constant number average molecular weight,the better the extensional viscosity characteristics.

Any of a variety of means of increasing the PLA's weight averagemolecular weight such that the degree of polylactide molecularentanglements increases, and therefore improvements of the extensionalviscosity characteristics of the polylactide polymer, may be used inaccord with the present invention.

introduction of Branching Into the Polymer Backbone

A method to improve the rheological properties of PLA is throughintroduction of branching into the polymer backbone. In particular, theintroduction of branching into the polymer backbone produces less linearpolylactide molecules. It is believed that less linear polylactidemolecules exhibit improved rheological properties because the molecularentanglements last longer due to decreased ability to move by reptation(diffusion). Reduced neck-in is one property improved with the lesslinear polymer's improved rheological behavior.

Generally, as illustrated in FIG. 2, linear polylactide polymers exhibita characteristic curve of intrinsic viscosity with respect to polymermolecular weight. As branching or other molecular interaction isintroduced into the PLA, the resulting curve of intrinsic viscosityversus molecular weight deviates significantly from the graph ofintrinsic viscosity versus molecular weight of a linear lactide polymer.This deviation is an indication that branching or other molecularinteraction has occurred.

A variety of techniques is available for introducing sufficientbranching into a linear polylactide to generate improved rheology. Forexample, an initiator may be used to introduce branching into PLA. Inyet another approach, non-initiating lactide reactants, such as anepoxidized hydrocarbon or an epoxidized oil, could be introduced intoPLA to form a branched (i.e. less linear) polylactide polymer. Asanother example, molecules containing at least two cyclic ester ringscould be copolymerized with lactide to form a branched (i.e. lesslinear) polylactide polymer. In this context the term "cyclic ester"includes any cyclic compound containing at least one ester group andcapable of ring opening polymerization. For example, cyclic esters mayinclude lactones, cyclic carbonates and cyclic oxalates.

Other techniques for introducing branching involve providing a reactantwhich will leave a residue unit in the PLA-based polymer that includes abulky organic group pendent therefrom. For example, a cyclic ester, suchas a long-alkyl chain (at least C₄) substituted lactone, could bereacted to form residue units in the polymer with the C₄ group pendenttherefrom.

The term "linear polylactide" as used herein refers to a linearnon-substituted polylactide polymer, such as those disclosed in U.S.Pat. Nos. 5,142,023, 5,247,058 and 5,247,059 to Gruber et al. The term"polylactide polymer" as used herein refers to a polymer in which themajority of repeat units in the polymer chains are lactic acid based orlactide based residues. For example, after removing additives such asfillers and plasticizers using methods known in the art, such asextraction and filtration, a polymer sample is hydrolyzed or saponified.Typically, a polylactide polymer, after removing additives, will yield50% or more, by weight, of lactic acid residues.

Providing Bridging Between the Polymer Backbones

Another way in which interaction between the polymer chains can beincreased is to introduce bridging between polymer backbones. Thisbridging can be introduced subsequent to polymer formation. Bridgingwill generally improve the extensional viscosity characteristics of thePLA by providing a small amount of cross-linking between the longbackbones and thus creating resistance to stretch or pull during polymermelt processing. Long backbone chains which have been bridged together,generally, form a new less linear polylactide molecule.

In general, as discussed previously, linear polylactide polymers exhibita characteristic curve of intrinsic viscosity with respect to thepolymer's molecular weight. As bridging is introduced into the linearpolylactide polymer, the resulting curve of intrinsic viscosity versusmolecular weight deviates significantly from the graph of intrinsicviscosity versus molecular weight of a linear polylactide polymer. Thisdeviation is an indication that bridging has occurred and that lesslinear polylactide molecules are present in the polymer.

Any of a variety of means can be used to determine the presence ofbranching of, or bridging between, polymer chains. The following is anexample of one technique. Control samples of dried and devolatilizedlinear polylactide are prepared. The molecular weights of the testsamples should be within the range of the molecular weights of thecontrols. The samples are then dissolved in a solvent. This solventshould be the same solvent that is used as a mobile phase for the gelpermeation chromatography (GPC). The intrinsic viscosity of each sampleis determined at the same temperature and in the same solvent as the GPCis run. Using GPC, the molecular weights of the samples should bedetermined relative to a standard, such as polystyrene. Either weightaverage molecular weight or viscosity average molecular weight is used.

Based upon the GPC results, a plot of the natural log of intrinsicviscosity (in deciliters per gram) versus the natural log of molecularweight should be made. In addition, a regression line should be made forthe control samples. This regression line is made by measuring themolecular weight and intrinsic viscosity of several (i.e. at least 3 andpreferably at least 7) linear polylactide samples and plotting theresults. These control samples should provide a range of molecularweights sufficient to accommodate the test samples as plotted on thesame chart as the regression line. The test sample is considered to havepreferred bridging or branching, in accord with the present invention,if the measured natural log of intrinsic viscosity is at least about 0.1below the predicted value based on the regression line for the controlsamples. More preferably, the sample is prepared such that the measurednatural log of intrinsic viscosity is at least about 0.2 below thecontrol line and even more preferably at least 0.4 below the controlline. See FIG. 2 for an example of a regression control line and asample point. With this bridging or branching, (i.e. molecularinteraction) the polylactide's viscosity is typically modified such thatphenomena, such as necking, will be significantly reduced. If the testsamples contain adulterants, such as plasticizers or fillers, theyshould be removed prior to determination of the extent to which there isbridging or branching in the polymer. Methods known in the art, such asdissolving, filtering and precipitating, can be used to remove theseadulterants.

Various techniques are available for providing bridging in the linearlactide polymer and thus converting it into a less linear lactidepolymer. For example, free radical generating peroxides can be used tocleave substituents from the polylactide backbones, generating a polymerradical that can bond with another polymer radical. Bridging may also beprovided through the reaction of multifunctional chain extenders, tolink polymer chains together and form a less linear polylactide.

Some Specific Means of Increasing Molecular Interaction

Overall, any means of increasing molecular interaction betweenpolylactide backbones such that the rheological properties of theresulting polymer are improved and the resulting polymer is useable inmelt processing operations, is in accord with the present invention.Generally, these means, as previously discussed, include increasingweight average molecular weight, providing branching in the polymerand/or providing bridging in the polymer. The following discusses somemore specific means of increasing molecular interaction betweenpolylactide polymer backbones. In general, control of the amount ofinteraction between the long polymer chains is desirable for maintaininga melt-stable, workable, compostable and/or biodegradable material. Inevaluating possible specific methods for improving rheologicalproperties, while at the same time retaining other preferredcharacteristics of melt stable polylactides, a number of approaches toincreasing interaction between long polymer chains of polylactide havebeen evaluated.

The principal approaches can be divided into two types. The first typeof approach involves reacting a radical generating moiety with a groupcontained in a polylactide polymer chain such that the residualpolylactide chain becomes a radical that can react with another residualpolylactide chain. Thus, two residual polylactide chains can bond orlink to one another. A variation of this approach involves using aradical generating moiety to link to a reactant having either a bulkyorganic group therein (for branching) or a functional group therein thatcan be later reacted to cause bridging, to the polymer. An example ofthis latter would be maleic anhydride.

The second principal approach involves including a moiety other thanunsubstituted lactide in some of the polylactide chains. There are atleast three types of moieties other than unsubstituted lactide that canbe included in the polylactide backbone in accord with the presentinvention. These three types of moieties originate from: a moleculecontaining one or more initiating groups; a molecule containing one ormore non-initiating reacting groups; and/or a molecule containing bothinitiating and non-initiating reacting groups. The term "initiatinggroup" refers to a moiety which can initiate polylactide chain formationby reacting with cyclic lactide in a ring opening reaction. The term"non-initiating reacting group" refers to a moiety with which lacticacid, lactide and/or the growing polylactide polymer can react duringpolymerization, but which does not itself initiate polylactide chainformation during the polymerization process (prior to its reaction withthe lactide acid, lactide or growing polylactide polymer). Thenon-initiating reacting group should also allow the polymer chain tocontinue propagating after it has reacted.

There are at least two types of molecules that contain initiatinggroups. The first type contains one group therein from which polylactidechain formation can be initiated during polymerization. The second typecontains more than one group therein from which polylactide chainformation can be initiated during polymerization. The compoundscontaining one and/or more than one of these types of groups arereferred to herein collectively as initiating reactants.

Similarly, there are at least two types of molecules that containnon-initiating reacting groups. The first type contains one grouptherein to which lactide can bond during polymerization, but which doesnot initiate polylactide chain formation through ring opening of cycliclactide (prior to reaction during polymerization). The second typecontains more than one group therein to which lactide can bond duringpolymerization, but which does not initiate polylactide chain formationthrough ring opening of cyclic lactide (prior to reaction duringpolymerization). Compounds containing one or more than one of thesetypes of groups are referred to herein collectively as non-initiatinglactide reactants.

Compounds containing molecules having both initiating and non-initiatingreacting groups therein can be used in accord with the present inventionand are referred to herein collectively as combination reactants. Inaddition, initiating reactants, non-initiating lactide reactants, and/orcombination reactants can be combined such that two or all three typesof reactants are included in the polymerization process.

The following are general architectures or configurations representingmolecules containing initiating reactants, non-initiating lactidereactants, and combination reactants. The first (1) configurationrepresents a molecule containing only one initiating group (X) therein.

    R--X                                                       (1)

The X represents the initiating group, and it can be any moiety whichcan initiate polylactide chain formation by reacting with cyclic lactidein a ring opening reaction. R represents any carbon containing groupthat does not prevent lactide polymer chain formation. R can be anon-linear carbon chain containing more than three carbon atoms.Preferably, R includes a bulky organic polymer entangling group therein.R can also contain conventional functional groups, which do not initiatelactide chain formation and which do not prevent the polymerizationprocess. The term "polymer entangling group" in this context is meant torefer to a group R or within R which is sufficiently bulky to facilitateentanglement of the resulting polymer molecule R-X-PLA with otherpolymer molecules, in the resulting polymer composition.

Configuration two (2) represents a lactide polymer formed from moleculessuch as depicted in configuration (1). In configuration (2), thecharacter PLA refers to a PLA-based polymer fragment which may (or maynot) include residues which are not from lactide or lactic acid.

    R--X--PLA                                                  (2)

The polymer molecule depicted in (2) can be representative of a moleculecontained in a polymer composition of the present invention. Preferably,R contains a non-functional polymer entangling group or non-linear groupthat facilitates molecular interaction such that the polylactidepolymer's elongational viscosity characteristics are improved relativeto linear polylactide of comparable weight average molecular weight. Inthis context, the word "improved" means that performance in amelt-processing operation is improved with respect to any of necking,bubble stability, reduced draw resonance or related characteristics.

Configuration (3) generally represents a molecule containing more thanone (e.g. three) initiating groups therein. ##STR1## The symbols X₁, X₂and X₃ each represent an initiating group. The chemical structure ofeach of these groups can be the same or different. The R represents anycarbon containing group that does not interfere with the polymerizationprocess as discussed previously. Of course, an initiating reactant doesnot necessarily have to include three functional groups. It only needsto have at least one. Configuration (3) is merely an example of aninitiating reactant molecule containing three initiating groups.

In general, configuration four (4) represents a lactide polymer moleculeformed from an initiating reactant such as the type represented inconfiguration (3). The term "PLA" in configuration (4) may be the sameas identified for configuration (2). ##STR2## The X₁, X₂, and X₃ inconfiguration (4) each represent the residual initiating groups, whichinitiated polylactide chain formation. Because the type of moleculedepicted in (4) is non-linear, it can facilitate molecular interaction.The polylactide chains formed from the initiating groups give greaterentanglement with polylactide chains contained in other molecules in thecomposition. Thus, polymerizing a lactide prepolymer mixture containingan initiating reactant, such as one represented in (3), can provide aless linear polymer with increased molecular interaction.

A molecule contained in a non-initiating lactide reactant is representedby configuration (5). ##STR3## In this configuration Y represents anon-initiating reacting group. R represents a carbon containing groupthat does not prevent lactide polymer chain formation, as discussedpreviously. Preferably, for non-initiating reactants R contains a bulkyorganic polymer entangling group containing more than three carbon atomsthat can entangle with other R groups and lactide polymer moleculesduring polymerization. If R is too large, then it can undesirablydiminish the flowability characteristics of the polymer.

In general, configuration six (6) represents a lactide polymer moleculeformed from a molecule such as represented by configuration (5). Inconfiguration (6), each PLA may be as defined for configuration (2).##STR4## The Y, in configuration (5) represents a residualnon-initiating group, which reacted with lactide and is incorporatedinto the resulting polymer chain. Due to the branching created by the Rgroup in (6), molecular interaction between backbone chains can beincreased, in accord with the present invention. From the aboveconfiguration (6), it will be apparent that preferably thenon-initiating group Y is a group which can react with lactic acid,lactide, or a growing PLA-chain and then which, upon reaction, forms anactive residue which can initiate further chain propagation. Asexplained hereinbelow, one such group is an epoxy group.

Configuration seven (7) represents a molecule containing anon-initiating lactide reactant having more than one (e.g. 2)non-initiating groups. ##STR5## The Y₁ represents one non-initiatingreacting group, and Y₂ represents a second non-initiating reactinggroup. Y₁ and Y₂ can be the same or different. For example, Y₁ can be anepoxy group, and Y₂ can be a cyclic ester containing group. As a furtherexample, Y₁ can be an epoxy group and Y₂ can be an epoxy group. Rrepresents a carbon containing group that does not prevent lactidepolymer chain formation, as discussed previously.

Configuration (8) represents a lactide polymer molecule formed from amolecule, such as configuration (7) containing two non-initiatingreacting groups. Each group (PLA) may be as defined for configuration(2). ##STR6## The Y₁ and Y₂ in configuration (8) represent the residualnon-initiating reacting groups which reacted with lactide and areincorporated into a polymer chain. The R group serves as a bridgebetween the backbones as two PLA-based polymer chains. This bridgingprovides increased molecular interaction such that the extensionalviscosity characteristics of the polymer can be improved.

Configuration nine (9) represents a molecule containing one initiatinggroup and one non-initiating reacting group. This molecule is of thetype that could be found in a combination reactant.

    Y--R--X                                                    (9)

The Y represents a non-initiating reacting group as above described, andthe X represents an initiating group as above described. The R, asdiscussed previously, represents any carbon containing group that doesnot prevent the polymerization process. The R contained in a combinationreactant, preferably, is one of the preferred types of groups discussedpreviously with respect to configurations (1)-(8).

Configuration (10) represents a lactide polymer molecule formed from amolecule such as the type disclosed in configuration (9). Each group(PLA) may be as defined for configuration (2). ##STR7## The Y and Xgroups in configuration (10) represent the residual non-initiatingreacting group and the residual initiating group. Because thepolylactide chains are oriented in to make the molecule in (10) lesslinear than linear polylactide, the opportunities for polylactide chainsto entangle with other chains is increased. This increased opportunityfor entanglement, generally, results in increased molecular interactionin the polymer.

It will be understood that the polymer molecules represented byconfigurations (2), (4), (6), (8) and (10) above could, and in typicalapplications will likely, contain more than one residue of the reactants(1), (3), (5), (7) and (9), respectively therein.

The following discussion is a detailed description of specific types ofradical generators, initiating reactants, non-initiating lactidereactants, and combination reactants that can be used in accord with thepresent invention. The specific initiating reactants, non-initiatinglactide reactants, combination reactants and resulting polymersdescribed or utilizing these reactants are of the general typesrepresented by configurations (1)-(10).

Generating Interaction Between Linear Polymer Molecules Using FreeRadical Reaction

This approach to generating small amounts of bonding between linearpolylactide molecules was generally characterized above. The followingscenario will provide a greater understanding of this technique.Consider a mixture of polylactide polymer materials. If a free radicalinitiator is provided in the mixture, the initiator will, uponactivation, generate free radicals. Among the possibilities of follow-upreaction, is that various free radicals from the initiator will reactwith carbon-hydrogen bonds in different polymer molecules, for exampleremoving a hydrogen atom from each and generating, in the remaining orresidual polymer molecule, a free radical. This reaction is believed tomost likely (statistically) take place at one of the tertiary carbons inthe polymer backbone.

The polymer has now become a free radical or a free radical residue of apolylactide polymer. Among the reactions of which it is capable, isreaction with yet another polymer molecule, which has been converted toa polymer radical by the same process. Reaction with the other polymerradical would generate a bond between the two polymer molecules. It willbe understood that in general such a polymer free radical reaction isstatistically unfavored. However, it need only occur to a small extentfor sufficient linear polymer linking (bridging) to occur, to increasemolecular interaction, and thus enhance rheology characteristics.

This mechanism for providing interaction among polylactide polymerchains, although useful, does have some drawbacks. For example, there ispotential for gel formation. More specifically, if too much initiator isused there may be so much interaction among the residual polymer chainsthat the polymer gels and loses much of its flowability characteristics.Processing a polymer with poor flowability characteristics can bedifficult and costly. Therefore, polymer gelling is discouraged.

The radical generator, preferably, is added during or after polylactideformation. Combining the radical generator with the polylactide afterpolymerization adds a step to the polymer processing. However, thereaction rate of this process is typically so fast that very littleadditional processing time is typically needed.

Another example of a drawback of this mechanism is that byproducts canbe produced. Because there is no precise control over what the radicalsgenerated during this process will react with, there are typicallyseveral types of byproducts that result from this reaction process.These byproducts may have to be separated from the resulting polylactidepolymer prior to melt-processing the polymer.

An advantage of this mechanism for generating interaction amongpolylactide chains is that many radical generators are inexpensive andreadily available. In addition, many break down to byproducts which arereadily removed, for example, by devolatilization. Also, the extent ofbonding is so small that the biodegradability or compostability of thepolylactide polymer is not significantly lost.

A variety of free radical initiators may be utilized to generateinteraction between linear polymer molecules according to thistechnique. In general, any radical initiator that readily removes amoiety, such as hydrogen, from a polylactide chain to form a residualpolylactide free radical (which can then react with another residualpolylactide free radical) can be used in accord with the presentinvention. A wide variety of peroxide radical initiators are known andcan be used. Peroxide initiators useable in accord with the presentinvention include: 2,5-dimethyl-2,5-di(t-butylperoxy) 3-hexyne;2,5-dimethyl-2,5-di(t-butylperoxy) hexane;2,5-dimethyl-2,5-di(t-amylperoxy) hexane;4-(t-butylperoxy)-4-methyl-2-pentanol;Bis(t-butylperoxyisopropyl)benzene; Dicumyl peroxide; Ethyl3,3-bis(t-butylperoxy) butyrate; Ethyl 3,3-bis(t-amylperoxy) butyrate;and, Dibenzoyl peroxide. Commercial products such as Lupersol 130;Lupersol 101; t-amyl 101; Lupersol D-240; Luperox 802; Luperox 500;Lupersol 233; Lupersol 533; and, Lucidol 78, available from ELF Atochemof Philadelphia, Pa. are useable. A preferred radical initiator is ethyl3,3-di-(t-butylperoxy)-butyrate), preferably as Luperco 233-XL(available from ELF Atochm, as a 40% concentration of the peroxide in aCaCo₃ carder). A preferred addition technique is to compound theperoxide into the PLA using a twin screw extruder.

In general, to achieve a sufficient interaction among polymer chains toimprove rheology (extensional viscosity characteristics) in a mannersufficient to inhibit necking or the like, a relatively large amount ofinitiator will be needed. Typically, if molar ratios of initiator topolymer of about 0.01:1 to 10:1 (more preferably 0.05/1 to 3/1) areused, a sufficient amount of polymer interaction will occur to achieveimprovement in rheology. In such circumstances (as has been observed)the number average molecular weight of the polymer increases by onlyabout ten percent, whereas the weight average molecular weight increasesabout twenty percent or more. Molar ratios of initiator to polymer ofabove about 10:1 are believed likely to cause excessive gelling intypical systems.

Generating Bridging using Chain Extenders

This approach to increasing molecular interaction is accomplished byproviding a chain extending agent. The chain extending agent bonds tothe terminus of the polymer chain. By providing a chain extending agentwith three or more functional groups a linear polylactide can be madeinto a less linear polylactide through bridging. If the polylactide isalready a less linear polylactide, then a bifunctional chain extendercan be used to increase molecular weight. Gel and network formation canbe a problem in this case, however.

Preferably, the chain extender will have a functionality of three orgreater. Typically, the extender should be present in a ratio of about0.1-1.0 equivalents of extender per mole of polymer.

Chain extenders can include any compounds capable of reacting with the--OH or --COOH terminus group. Examples include oxazolines, isocyanates,dihydrooxazines, and anhydrides. Preferred chain extenders would benon-toxic and biodegradable.

Providing an Initiating Reactant

One of the means of increasing molecular interaction between linearpolylactide chains is to provide an initiating reactant, into theprepolymer or polymerizing mixture, from which lactide polymer chainscan grow during polymerization. As discussed previously, the moleculescontained in the initiating reactant can have one initiating group fromwhich lactide polymer formation can begin or more than one initiatinggroup from which lactide polymer formation can begin. However, if themolecules contain only one or two initiating groups, an additional meansof providing interaction will likely have to be provided in order toincrease molecular interaction. In other words, providing one or twoinitiating groups alone may not increase molecular interaction betweenpolymer backbones, because usually bridging or branching cannot beintroduced using these types of initiating reactants.

However, if a combination reactant, which includes molecules having atleast one initiating group and at least one non-initiating reactinggroup, is used, for example, molecular interaction can be increased inthe resulting polymer, although there is only one or two initiatinggroups. This was discussed with respect to configurations (9) and (10).In sum, reactants containing molecules having one or two initiatinggroups can be used to increase molecular interaction among polylactidebackbone chains, depending upon what else is in the molecule.

More specifically, if an initiating reactant molecule having a singleinitiating group thereon is utilized, for example, a single polymerchain begins to form from the initiating group, such as an --OH groupcontained in the initiating reactant molecule, during lactidepolymerization. However, this single chain alone generally does notincrease molecular interaction between the backbone chains (unless theinitiating reactant includes an appropriate pendent group therein).Therefore, unless the initiating reactant molecule contains asufficiently bulky organic group therein, the initiating reactantmolecule should include therein a non-initiating reacting group, forexample, such as an epoxide, in order to increase molecular interactionby providing branching and/or bridging; thus, making this initiatingreactant a combination reactant.

An initiating reactant containing molecules that have more than oneinitiating group thereon can be used to increase the molecularinteraction between linear polylactide chains. However, if this type ofinitiating reactant molecule contains only two initiating groupsthereon, such as two active --OH groups, the polymer can begin to growin two directions from the initiating group (i.e., growth will beginfrom each initiator). Therefore, a linear polymer can result asdiscussed previously with respect to use of an initiating reactantmolecule having only one initiating group, and molecular interactionbetween backbones is not increased (unless the initiating reactantincludes one appropriate bulky pendent group therein or is a combinationreactant). Thus, as discussed with respect to the initiating reactanthaving one initiating group, an additional means of increasing molecularinteraction can be used, or the reactant molecule can be appropriatelyconfigured, so molecular interaction can be increased. For example, thereactant molecule may be a branched polymeric molecule.

If the initiating reactant molecule includes three or more initiatinggroups, such as --OH groups, thereon, for example, long polymer chainscan begin to grow in at least three directions from the initiatingreactant molecule. In essence, each initiating group could provide asingle point of branching, at a terminus of each of three or more, long,polymer chains, such as previously discussed with respect toconfigurations (3) and (4). Thus, the result of utilization of aninitiating reactant with three or more active groups can be introductionof a small amount of interaction between long polymer chains, and thusimprovement in extensional viscosity characteristics and improvement inrheology characteristics with respect to necking. These initiatingreactants can be added either before or during polymerization of lactideas described in U.S. Pat. Nos. 5,247,059 and 5,274,073 issued to Gruberet al., for example. These reactants can include molecules (or evenreactive groups in one molecule) of one type or a mixture of severaltypes depending upon the particular polymer desired.

A variety of types of initiating reactants can be used in accord withthe present invention. Any initiating reactant molecule with one or moreinitiating groups that can be used to initiate polylactide chainformation is useable in the present invention as long as the resultingpolymer is of a sufficient molecular weight for the particularapplication in which it is to be used. Preferably, the resulting polymeris melt stable. Typically, these groups will be --OH (hydroxy) or NH₂(amine) groups. In addition, it would also be preferable that theseinitiating reactants be biodegradable. Some examples of initiatingreactants that can be used in accord with the present invention aresugars, alcohols such as dodecanol; diols such as 1,6-hexanediol;hydroxy esters such as methyl lactate; glycerol;2-ethyl-2(hydroxymethyl)-1,3 propane diol; pentaerythritol;di-pentaerythritol; erythritol; xylitol; and, sorbitol. The latter 7compounds are preferred since they have a reactive functionality of ≧3.

During polymerization, the lactide chains grow from the initiatinggroups. Thus, lactide polymer chains (i.e. PLA-based polymers),containing the residue(s) of an initiating group result. This resultingpolymer will have a substantially improved extensional viscositycharacteristics and melt flow properties and will be less linear. Inaddition, for lactide polymer chains formed from an initiating reactant,there typically is relatively little gel formation. Lactide polymersproduced by some of the other methods in accord with the presentinvention significantly gel, which is undesirable and which can increasethe viscosity such that the polymer can lose its flowabilitycharacteristics.

Lactide tends to react very quickly with the initiating groups.Therefore, the reaction time for polymerization using initiatingreactants can be short. Typically, the amount of initiating reactantthat is needed to generate the less linear polymer with improvedextensional viscosity characteristics varies with the particularapplication of the polymer. In general, enough initiating reactantshould be added to the prepolymer or polymerizing mixture in order forthere to be enough molecular interaction, such as entanglement of thelactide polymer chains, to improve the extensional viscositycharacteristics and improve melt flow properties. However, there cannotbe so much initiating reactant added that the molecular weight of thepolymer is reduced below a critical molecular weight. In general, morethan about 5% of the polymer composition by number should be moleculescontaining residues of initiating reactant molecules. Preferably, theconcentration of molecules containing residues of initiating reactantmolecules is more than about 20% by number. Most preferably, theconcentration of molecules containing residues of non-reactiveinitiating reactant molecules is more than about 33% by number.Preferably, a resulting polymer in accord with the present invention hasa weight average molecular weight of at least about 100,000.

Unfortunately, this approach to developing interaction between thepolymer chains of an inherently linear polymer such as polylactide isnot always fully satisfactory in practical application. In general, forcommercial exploitation, polylactides can be formed from thepolymerization of lactide purified according to the methods described inU.S. Pat. Nos. 5,142,023 and 5,247,059 to Gruber et al; i.e.purification procedures which do not involve recrystallization of thelactide mixture from a solvent. Such lactide, while relatively pure, caninclude significant amounts of initiators, which are byproducts of thelactide production process, therein. For example, some of theseinitiators have an active --OH group. While the amounts of byproducts inthe purified lactide can be effectively reduced to zero by extremepurification methods (such as recrystallization from a solvent), ingeneral, it is not necessarily commercially practical to engage inexcessive purification efforts. These byproducts include, for example,water and lactic acid, which can compete with the initiating reactantsadded to the prepolymer mixture, in almost any commercially feasible,large-scale, polymerization process. This restricts the concentration ofmolecules containing residues of the added initiating reactants.

A net result of utilizing initiating reactants, such as hydroxylinitiating reactants in the presence of substantial amounts of reactiveinitiators, which are reaction byproducts or intermediates, such aslactic acid and water, can be a reduction in the molecular weight of theresulting polymer. The result can be a less desireable polymer, withrespect to its melt processing characteristics. In particular, theaddition of initiating reactants to the prepolymer or polymerizingmixture can promote a reduction in the weight average molecular weight.A reduction in the weight average molecular weight can result in adecrease in the amount of molecular interaction, such as molecularentanglements; and therefore, the film forming properties of thepolylactide can be compromised, despite the introduced branching.

In sum, while initiating reactants can be utilized to generatepolylactide polymers, which are modified to have some interaction amongthe long polymer chains, in practice the approach is not always fullydesirable. In order to control polymer molecular weight in a desiredmanner, it would be desirable to have very strict control of theco-presence of reactive initiators (such as water, lactic acid oroligomers) in the materials. In commercially useable purified lactides(not purified by recrystallization from a solvent), a sufficient amountof residual water and/or lactic acid or oligomers is usually present, toprovide for a level of single chain initiation which is such thatundesirable polymers, with respect to molecular weight, can result ifinitiating reactants are also added to the polymerization process. Thus,for use with present commercially viable processes for purifying lactide(especially those which do not involve recrystallization from asolvent), alternate approaches described herein to providing polylactidehaving some interaction among the long polymer chains, typically, willbe preferred.

Providing Non-initiating Lactide Reactants

Another approach to increasing molecular interaction involves utilizingnon-initiating lactide reactants to generate interaction between longpolymer chains. This technique is advantageous because it does notinvolve the addition of initiating reactants into the prepolymer orpolymerizing mixture. Thus, it is well adapted to application inprocessing using polylactide mixtures which have not been purified byrecrystallization from a solvent.

In general, a non-initiating lactide reactant is a material which, whenreacted with lactic acid, lactide or polylactide, reacts with an active--OH in the polylactide but which cannot, by itself and before reactionwith the lactic acid, lactide or polylactide, initiate propagation. Forexample, for propagations involving lactide ring opening to formpolylactides, epoxy compounds are non-initiating lactide reactants. Inparticular, when the active --OH group of a lactide or polylactidemolecule reacts with the epoxy group contained in a non-initiatinglactide reactant, the oxirane ring opens and provides a new --OH groupfor further reaction with lactides (i.e. chain propagation). However,for each oxirane group only one reactive --OH group (for propagation) isformed from a reaction with the lactic acid or lactide polymer. Thus,the oxirane ring does not initiate polymer formation but rather merelybecomes incorporated into the polymer chain and will permit chainpropagation to continue.

In general, if the non-initiating lactide reactant has essentially onenon-initiating group, such as an oxirane ring or an epoxy group thereon,the net result is the formation of a polymer of linear molecules eachhaving one or more residues of the non-initiating lactide reactantmolecule incorporated therein. If the non-initiating lactide reactantalso has a polymer entangling group thereon, such as a polyester,polyether or hydrocarbon, then these pendent chains can entangle withpolylactide chains and/or other entangling groups to increase molecularentanglement and therefore increase molecular interaction and improvemelt flow properties. It is believed that if the groups in the pendentchains are such that they comprise at least about 10% by weight of thepolymer, the melt flow properties will be significantly andadvantageously altered. These pendent chains could be provided either asa large number of short chains or as a few long chains. This mechanismwith respect to a single epoxy group is also applicable to othernon-initiating lactide reactant molecules containing only onenon-initiating reacting group. The determination of at least 10% wouldtypically be done based on reactants.

If the non-initiating lactide reactant molecule includes twonon-initiating reactive groups, such as cyclic esters, or epoxy groupsthereon, it can be used to link long polymer chains together (i.e., theresidue of the non-initiating lactide reactant molecule becomes abridge). The bridge can be longer if the active groups are at the endsof a hydrocarbon chain, for example. Similarly, if a non-initiatinglactide reactant includes three or more non-initiating reactive groupsthen the result can be a polymer molecule having numerous longpolylactide chains extending in different directions. In general, theuse of non-initiating lactide reactants leads to a polymer with improvedmelt flow properties and preferred characteristics with respect toprocessing phenomena, such as necking.

A variety of materials are useable to generate improved polylactidepolymers with respect to melt flow properties, through reaction withnon-initiating lactide reactants. Useful non-initiating lactidereactants for this purpose include, for example, copolymerizing agentshaving one epoxy group per molecule and a bulky organic group such as ahydrocarbon chain containing at least four carbon atoms. Other usefulnon-initiating lactide reactants include, for example, copolymerizingagents having two or more epoxy groups per molecule, such as manyepoxidized oils. When copolymerizing agents containing a hydrocarbonchain of at least four carbon atoms and having at least one epoxy groupper molecule are added before or during polymerization, a less linearpolymer can result when compared to non-copolymerized lactide polymers.Also, when copolymerizing agents having two or more epoxy groups permolecule are added to the prepolymer mixture before or duringpolymerization, a less linear polymer can result when compared tonon-copolymerized lactide polymers.

Useful copolymerizing agents or non-initiating lactide reactants havingepoxide groups include epoxidized fats and oils of many kinds. Inparticular, when lactide is copolymerized with an epoxidized oil, it isbelieved that the oxirane rings of the epoxidized oil react with eitherterminal alcohol groups or terminal acid groups of the lactide polymerduring reaction to form a less linear lactide polymer compared to anon-copolymerized lactide polymer.

Preferably, epoxidized: fatty acids, glycerides, diglycerides,triglycerides and mixtures thereof, are used a copolymerizing agents.More preferably, epoxidized: animal fats, animal oils, vegetable fats,vegetable oils, monoglycerides, diglycerides, triglycerides, free fattyacids and derivatives thereof are used. Most preferably, epoxidizedvegetable oils such as epoxidized linseed oil, epoxidized soybean oiland mixtures thereof are used. Additional useful epoxidized oils mayinclude epoxidized: cottonseed oil, ground nut oil, soybean oil,sunflower oil, rape seed oil or cannola oil, sesame seed oil, olive oil,corn oil, safflower oil, peanut oil, sesame oil, hemp oil, tung oil,neat's food oil, whale oil, fish oil, castor oil, and tall oil.

Epoxidized linseed oil has been used as a copolymerizing agent withgreat success. In particular, an epoxidized linseed oil known as Flexol®Plasticizer LOE (commercially available from Union Carbide Corporation)is a preferred copolymerizing agent of the present invention.

It is interesting that epoxidized linseed oil is marketed as aplasticizer, however the T_(g) of the resultant polymer is fairyconstant, which indicates little plasticizing effect at the levelstested. An advantage associated with copolymerizing agents such asepoxidized linseed oil, is they can act as a lubricant during processingwithout the resultant polymer having a greasy texture.

Epoxidized soybean oil, for example, Paraplex® G-62, commerciallyavailable from C. P. Hall Corp., is also a preferred copolymerizingagent for the present invention.

It has been found that die processability characteristics can beimproved with use of compositions and methods of the present invention.In particular, it has been found that, when processing polymers of thepresent invention while holding temperature, molecular weight, polymerflow rate and plasticizer concentration constant, there can be areduction in die pressure when compared with linear non-functionalizedpolylactide polymers of comparable weight average molecular weight. Thisadvantageous reduction in die pressure has been found to be most evidentwhen using non-initiating lactide reactants, such as epoxides, topromote molecular interaction in accord with the present invention.

Coating operations, for example, can be conducted more efficiently withuse of a polymer that contributes to improved die processabilitycharacteristics, such as reduced die pressure. This reduction can saveenergy and reduce equipment wear. Preferably, in accord with the presentinvention, a polymer is prepared such that it can be processed with adie pressure that has been reduced at least 10% when compared withlinear non-functionalized PLA of comparable weight average molecularweight that is processed under similar conditions. More preferably, thepolymer is prepared such that there has been at least a 15% die pressurereduction and most preferably, there has been at least a 20% diepressure reduction. In general, a preferred polymer in accord with thepresent invention is prepared such that it can be processed with a diepressure that has been reduced with respect to a linear polylactide ofcomparable weight average molecular weight this is melt processed underthe same conditions. This die pressure reduction is illustrated below inExamples 9 and 13.

Regardless of what type copolymerizing agent (i.e. non-initiatinglactide, reactant) is used, the amount of copolymerizing agent added tothe prepolymer mixture can vary with the specific application.Generally, if the amount of copolymerizing agent (i.e. non-initiatinglactide reactant) added to the prepolymer or polymerizing mixture isinsubstantial, then the melt flow properties of the resulting polymerwill not be improved. Moreover, if too much copolymerizing agent (i.e.non-initiating lactide reactant) is added to the prepolymer orpolymerizing mixture then the reaction can lead to very high molecularweight polymers and/or gels. In general, the amount of copolymerizingagent will vary with the desired molecular weight and polydispersityindex of the resulting polymer. A practical lower limit on thecopolymerizing agent is to have at least 1 equivalent (equivalents=molesof functionality) of copolymerizing agent for every 20 moles of polymer.More preferably, the copolymerizing agent is present at a level of atleast 1 equivalent of copolymerizing agent for every 10 moles ofpolymer. Most preferably, the copolymerizing agent is present at a levelof at least 1 equivalent of copolymerizing agent for every 5 moles ofpolymer.

A practical upper limit on the copolymerizing agent is determined basedon the following conservative estimate of a theoretical gel point (TGP).The TGP, in equivalents of copolymerizing agent per mole of polymer, isestimated as:

    TGP=f/f-1

where f is the functionality of the copolymerizing agent. Theconcentration of copolymerizing agent is preferably less than 5×TGP,more preferably less than 2×TGP, and most preferably less than 1×TGP.The moles of polymer can be estimated beforehand from the total moles ofinitiator, as determined, for example, by gel permeation chromatography.

For f=1 the TGP goes to infinity, as gelation cannot occur. For thiscase, the maximum amount of copolymerizing agent is preferably less than50%; and more preferably less than 10% of the polymer weight.

Preferably, the copolymerizing agent is biodegradable, or forms abiodegradable residue in the polymer, so that combinations of thelactide and copolymerizing agent (i.e. non-initiating lactide reactant)can also be biodegradable.

In addition to epoxides, cyclic esters can be used as non-initiatinglactide reactants. Cyclic esters, such as lactones, can be used asnon-initiating lactide reactants. As discussed above with respect toepoxides, if a non-functional cyclic ester is used, then thenon-initiating reacting group should preferably be at the terminus of anon-functional polymer entangling group contained therein. If amultifunctional cyclic ester is used, for example, then thenon-initiating reacting group(s) of the multifunctional cyclic esterserves as a bridge between two polylactide chains. Either of thesemechanisms can provide significant branching or bridging and thusincreased molecular interaction in the polymer.

A variety of cyclic esters that react with lactic acid, lactide orgrowing lactide polymer, (without initiating or terminating propagation)can be used as a non-initiating lactide reactant, as long as theresulting polymer's processability is not compromised. Thesenon-initiating lactide reactants can be added before or duringpolylactide polymerization.

Preferably, the cyclic esters or lactones used as non-initiating lactidereactants are ones which are biodegradable in accord with thebiodegradability of the resulting polylactide polymer.

Multi-cyclic esters may also be used as non-initiating lactidereactants. For example, bis-2,2-(E-caprolacton-4-yl) propane is useable.Other useable multi-cyclic esters are identified in U.S. Pat. No.3,072,680, the disclosure of which is incorporated herein by reference.

A drawback of using cyclic esters is that they presently can berelatively expensive. However, an advantage is these reactants can reactat about the same rate as lactide during polymerization so that apredictable, uniform, branched polymer can be produced.

Any of the compounds and/or methods described in this section can becombined in order to form a viscosity modified polylactide polymer. Forexample, more than one type of non-initiating lactide reactant can beadded to the prepolymer mixture in order to form a polymer that containsresidues of more than one type of non-initiating lactide reactant.Chemically different types of non-initiating lactide reactants can becombined and added to the prepolymer mixture (e.g. both a reactantcontaining a cyclic ester and a reactant containing an epoxide can beused). In addition, both non-initiating lactide reactants and initiatingreactants can be added to the prepolymer mixture. Further, a combinationreactant can be used instead of an initiating reactant and/ornon-initiating lactide reactant in accord with the present invention.Moreover, a combination reactant can be combined with an initiatingand/or non-initiating lactide reactant to promote molecular interactionin accord with the present invention.

Utilizing Initiating Reactants and/or Non-initiating Lactide ReactantsContaining Groups Capable of Controlled Reaction

Another means of promoting molecular interaction between linearpolylactide chains is provided by using selected initiating reactantsand/or non-initiating lactide reactants having therein groups which arenot themselves initiating or non-initiating during the polymerizationreaction, but rather are groups which can be later reacted to linklinear polymer chains. For example, if a reactant molecule contains amoiety having one active hydroxy group therein (i.e., an initiatinggroup) and located elsewhere in the initiating reactant molecule is acarbon-carbon double bond, the following scenario is possible.

The initiating group can be utilized to, through reaction of the activehydroxy group, generate formation of a single linear polymer chainhaving located near one terminus the carbon-carbon double bond moiety.As a result, when such an initiating reactant molecule is used, theresulting polymer mixture will include therein various polymer moleculeshaving a carbon-carbon double bond near a terminus thereof. These doublebonds can be reacted through a variety of processes, in a controlledmanner, to "link" the linear polymer molecules to one another. Forexample, the carbon-carbon double bonds can be reacted by a subsequentfree radical reactions with or without an additional monomer capable offree radical polymerization. As another example, a non-initiatinglactide .reactant may have an epoxy group at one terminus of ahydrocarbon chain and a carbon-carbon double bond at another terminus ofthe chain. In this latter case, the carbon-carbon double bond would belocated in a group pendent from a residue of the non-initiating lactide,somewhere in the resulting PLA-based polymer molecule.

Although the method of making a lactide polymer using a reactantcontaining an epoxide group, for example, and a carbon-carbon doublebond as the non-initiating lactide reactant, requires an additionalprocessing step, these types of non-initiating lactide reactants arereadily available and provide for gel and reaction control. Thisnon-initiating lactide reactant would be added either before or duringpolymerization of the lactide polymer. Once the epoxide is reacted, thecarbon-carbon double bond would provide a means for additionalpolylactide moieties to react. Thus, the double bonds could be reacted,as described below, to provide bridging between the polymer.

Useable non-initiating reactants which include a carbon-carbon doublebond are identified at cols. 7-8, in U.S. Pat. No. 4,644,038,incorporated herein by reference, and include: 1,2-epoxy-7-octene;glycidyl acrylate; glycidyl methacrylate; glycidyl undecylenate; allylglycidyl ether; methyl vinyl glycidyl amine; vinyl 3,5-epoxycyclohexane; allyl 3,4-epoxy cyclohexane; 3,4-epoxy-cyclohexyl acrylate;2,3-epoxypropyl 4-vinyl phenyl ether; 2,3-epoxy cinnamyl acrylate;2,3-epoxybutyl methacrylate; and, 9,10-epoxyoleyl acrylate.

In general, preferred initiating reactants and non-initiating lactidereactants for such an application of principles, in accord with thepresent invention, will include those initiating reactants andnon-initiating lactide reactants that have a (preferably) terminalhydroxy, amine, and/or epoxy group therein and also a (preferably)terminal carbon-carbon double bond or other potentially reactive moiety.Reaction rate and gel formation are better controlled if these types ofinitiating reactants and/or non-initiating lactide reactants are used.In addition, many of these types of initiating reactants andnon-initiating lactide reactants are readily available. These initiatingreactants and non-initiating lactide reactants can be added before orduring polymerization, but a reaction step is needed to react thepotentially reactive moiety, such as the double bonds. If double bondsare provided they can be reacted by any means known in the art, such aswith use of a free radical process. This post reaction step isrelatively fast so reaction residence time will not be significantlyincreased.

The amount of initiating and/or non-initiating lactide reactantscontaining a double bond to be added varies with the particularapplication and type of reactant used. In general, enough initiatingand/or non-initiating lactide reactant should be added such that theextensional viscosity of the polymer is sufficiently improved. However,not so much initiating reactant and/or non-initiating lactide reactantshould be added such that the polymer loses its flowabilitycharacteristics and becomes difficult to process. In general, about 0.01to about 0.30 equivalents of initiating reactant and/or non-initiatinglactide reactant should be used per mole of polymer and preferably about0.02 to about 0.15 mole/mole.

In order to protect the unsaturated bond during polymerization a freeradical inhibitor should be used. Some types of free radical inhibitorsuseable in accord with the present invention include: the quinones (e.g.p-benzoquinone; hydroquinone; 2,5-dihydroxy-p-benzoquinone;1,4-naphthoquinone; and 2,5-diphenyl-p-benzoquinone); aromatic-nitrogencompounds; trinitro benzene; sulfur; ammonium thiocyanate;dinitrochlorobenzene; 2,2-diphenyl-1-picrylhydrazyl; metal halides;2,6-di-t-butyl cresol; quaternary ammonium halides; picric acid;chloranil; 4-amino-1-napthol; copper; and, copper compounds. Preferablythe quinones without hydroxy groups are used. The residues of the freeradical inhibitor, initiating reactant, and/or non-initiating lactidereactant used in this mechanism, if possible, should be biodegradable sothat the resulting polylactide polymer does not lose itsbiodegradability. The amount of radical inhibitor varies with thereaction conditions. If too little radical inhibitor is added to theprepolymer mixture, then a significant portion of the bonds will not beprotected. If too much radical inhibitor is added, then subsequentreaction may be difficult to initiate. The appropriate amount ofinhibitor, for any given system, can be readily determined by one ofskill, by experimentation. In general, an inhibitor concentration ofabout 0.01 to 1.0 weight percent, based on weight of the carbon-carbondouble bond containing reactant is suitable.

Preparation of Improved Melt Stable Lactide Polymers

In general, lactide polymers according to the present invention aremanufactured from the polymerization of lactide. Except for theimprovements defined herein with respect to interacting long polymerchains for rheology improvement, general techniques for preparation oflactide polymers according to the present invention are disclosed inU.S. Pat. Nos. 5,142,023 and 5,247,059 to Gruber et al. Thus, thetechniques described herein are well adapted for use in continuousprocessing and are not limited to use in batch processing. Thesetechniques may be applied, with modifications as described herein, toobtain improved polymers according to the present invention.

In general, various techniques outlined above for generating interactionamong linear polymers can be characterized as practiced in at least oneof three general manners: by providing a reactant or initiator in theprepolymer mixture prior to polymerization; providing a reactant orinitiator during lactide polymerization, or possibly by providing areactant or initiator after lactide polymerization. An example of thefirst type of modification is the general technique of providing aninitiating reactant in the prepolymer mixture. An example of the secondtechnique is providing a non-initiating lactide reactant in the lactidemixture during polymerization. An example of the third technique isutilization of a free radical initiator to create polymer radicals whichreact to generate bonding between polymer molecules, afterpolymerization.

Melt-Stable Polymers Generally

The preferred lactide polymers of the present invention are melt-stable.By "melt-stable" it is meant generally that the lactide polymer, whensubjected to melt-processing techniques, adequately maintains itsphysical properties and does not generate by-products in sufficientquantity to foul or coat processing equipment. The melt-stable lactidepolymer exhibits reduced degradation relative to conventional lactidepolymers. It is to be understood that degradation will occur duringmelt-processing. The compositional requirements and use of stabilizingagents reduces the degree of such degradation to a point where physicalproperties are not significantly negatively affected by melt-processing,and fouling by impurities or degradation by-products does not occur.

Furthermore, the melt-stable polymer should be melt-processable inmelt-processing equipment such as that available commercially. Further,the polymer should retain molecular weight and viscosity. The polymershould have sufficiently low viscosity at the temperature ofmelt-processing so that the melt-processing equipment may mechanically,for example, extrude the lactide polymer in a polymer processingoperation. The temperature at which this viscosity is sufficiently lowshould also be below a temperature at which substantial degradationoccurs.

A standard test for determining whether a lactide polymer is melt-stableincludes placing a small portion of a devolatilized sample of lactidepolymer in a closed vial and placing such vial in a 180° C. oil bath. Asample is taken at times of 15 minutes and 1 hour. A melt-stable lactidepolymer will show formation of less than 2 percent lactide in the15-minute sample and, more preferably, formation of less than 2 percentlactide in the 1-hour sample. It is more preferable that the stabilizedlactide polymer form lactide contents of less than 1 percent in both the15-minute and 1-hour samples. At equilibrium there is a concentration of3.6 weight percent lactide at 180° C.

The melt-stable lactide polymer composition may include other polymericspecies which can, for example, be incorporated through melt blending.Examples of other polymers which could be blended include, but are notlimited to, poly(hydroxybutyrate); poly(hydroxybutyrate-co-hydroxyvalerate); poly(vinyl alcohol); poly(caprolactone); and,poly(glycolide). Preferably, the blended polymer is biodegradable,compostable, and made from annually renewable resources.

Polymer Composition

Preferred melt-stable lactide polymer compositions of the presentinvention comprise a mixture of polylactide polymer chains having anumber average molecular weight from about 10,000 to about 300,000. Morepreferably, the number average molecular weight is at least 50,000. Instill more preferred compositions, the number average molecular weightranges from about 50,000 to about 150,000. In general, physicalproperties such as modulus, tensile strength, percentage elongation atbreak, impact strength, flexural modulus, and flexural strength remainstatistically constant when the lactide polymer samples are above athreshold molecular weight. The lower limit of molecular weight of thepolymer compositions of the present invention is preferably above about50,000 in order to result in a lactide polymer with predictable physicalproperties upon melt-processing. There typically is a practical upperlimit on molecular weight based on increased viscosity with increasedmolecular weight. In order to melt-process a high molecular weightlactide polymer, the melt-processing temperature should be increased toreduce the viscosity of the polymer. The exact upper limit on molecularweight should be determined for each melt-processing application in thatrequired viscosities vary and residence time within the melt-processingequipment will also vary. Thus, the degree of degradation in each typeof processing system will also vary. It is believed that one coulddetermine the suitable molecular weight upper limit for meeting theviscosity and degradation requirements in any application.

Preferably, the polymer is prepared to have a weight average molecularweight of at least about 100,000 and not greater than 1,200,000. Themelt-stable lactide polymer compositions in a preferred embodiment aredependent on the desired crystalline state of the product. For asemi-crystalline product the polymer compositions are the reactionproduct of polymerizing a lactide mixture comprising about 15% by weightor less of meso and D-lactide, with the balance L-lactide. Morepreferably, the reaction mixture will contain less than 6% by weight ofmeso and D-lactide, with a balance of L-lactide. For an amorphousproduct, the polymer compositions are generally the reaction product ofpolymerizing a lactide mixture comprising about 6% by weight or more ofmeso-and D-lactide, with a balance of L-lactide. More preferably, thereaction mixture will contain more than about 9% but less than about 50%by weight of meso-and D-lactide, with the balance L-lactide. The opticalcomposition disclosed includes the benefit of utilizing meso-lactide asdisclosed by Gruber et al. in U.S. application Ser. No. 07/955,690 whichwas filed on Oct. 2, 1992 which is now U.S. Pat. No. 5,338,822 is herebyincorporated by reference.

In accord with the present invention, the prepolymer mixture (i.e.lactide monomer) may contain additional cyclic ester monomers along withlactide. For example, dioxanones (such as p-dioxanone), lactones (suchas e-caprolactone or 4-valerolactone), dioxan(dione)s (such as glycolideor tetramethyl 1,4-dioxan-2,5-dione), or ester-amides (such asmorpholine-2,5-dione).

The residual monomer concentration (if any) in the preferred melt-stablelactide polymer composition is less than about 2 percent by weight. In apreferred composition the concentration of residual lactide monomer inthe polymer is less than about 1 percent by weight and more preferablyless than about 0.5 percent by weight. It has been found that themonomer should not be used as a plasticizing agent in the resin of thepresent invention due to significant fouling or plating out problems inprocessing equipment. It is believed that, typically, low levels ofmonomer concentration do not plasticize the final polymer.

The water concentration (if any) within the melt-stable lactide polymercomposition preferably is less than about 2,000 parts-per-million. Morepreferably, this concentration is less than about 1000 parts-per-millionand most preferably less than 500 parts-per-million. The polymermelt-stability is significantly affected by moisture content. Thus, themelt-stable polymer should have the water removed prior tomelt-processing. It is recognized that water concentration may bereduced prior to processing the polymerized lactide to a resin. Thus,moisture control could be accomplished by packaging such resins in a waywhich prevents moisture from contacting the already-dry resin.Alternatively, the moisture content may be reduced at themelt-processor's facility just prior to the melt-processing step in adryer. It has been found that the presence of water can cause excessiveloss of molecular weight which may affect the physical properties of themelt-processed polymer.

In preferred compositions of the present invention, a stabilizing agentof a type and in an amount sufficient to reduce yellowing and molecularweight loss is included in the melt-stable composition. Stabilizingagents useful in the present polymer compositions comprise antioxidantsand/or water scavengers. Preferred antioxidants are phosphite-containingcompounds, hindered phenolic compounds or other phenolic compounds.Useful antioxidants include such compounds as trialkyl phosphites, mixedalkyl/aryl phosphites, alkylated aryl phosphites, sterically hinderedaryl phosphites, aliphatic spirocyclic phosphites, sterically hinderedphenyl spirocyclics, sterically hindered bisphosphonites, hydroxyphenylpropionates, hydroxy benzyls, alkylidene disphenols, alkyl phenols,aromatic amines, thioethers, hindered amines, hydroquinones and mixturesthereof. Commercially-available stabilizing agents have been tested andfall within the scope of the present melt-stable lactide polymercomposition. Biodegradable antioxidants are preferred.

The water scavengers which may be utilized in preferred embodiments ofthe melt-stable lactide polymer composition include: carbondimides,anhydrides, acyl chlorides, isocyanates, alkoxy silanes, and desiccantmaterials such as clay, alumina, silica gel, zeolites, calcium chloride,calcium carbonate, sodium sulfate, bicarbonates or any other compoundwhich ties up water. Preferably the water scavenger is degradable orcompostable.

In the manufacture of the melt-stable lactide polymer compositions ofthe present invention, the reaction to polymerize lactide is typicallycatalyzed. Many catalysts have been cited in literature for use in thering-opening polymerization of lactones. These include but are notlimited to: SnCl₂, SnBr₂, SnCl₄, SnBr₄, aluminum alkoxides, tinalkoxides, zinc alkoxides, SnO, PbO, Sn (2-ethyl hexanoates), Sb(2-ethyl hexanoates), Bi (2-ethyl hexanoates), Na (2-ethyl hexanoates)(sometimes called octets), Ca stearates, Mg stearates, Zn stearates, andtetraphenyltin. Applicants have also tested several catalysts forpolymerization of lactide at 180° C. which include: tin(II) bis(2-ethylhexanoate) [T-9, Atochem], dibutyltin diacetate [Fascat 4200®, Atochem],butyltin tris(2-ethyl hexanoate) [Fascat 9102®, Atochem], hydratedmonobutyltin oxide [Fascat 9100®, Atochem], antimony triacetate [S-21,Atochem], and antimony tris(ethylene glycoxide) [S-24, Atochem]. Ofthese catalysts, tin(II) bis(2-ethyl hexanoate), butyltin tris(2-ethylhexanoate) and dibutyltin diacetate appear to be most effective.

It has been found that the use of catalysts to polymerize lactidesignificantly affects the stability of the resin product. It appears thecatalyst as incorporated into the polymer also is effective atcatalyzing the reverse depolymerization reaction. To minimize thisnegative effect, in preferred compositions, the residual catalyst levelin the resin is present in a molar ratio of monomer-to-catalyst greaterthan 3,000:1, preferably greater than 5,000:1 and most preferablygreater than 10,000:1. It is believed that a ratio of 20,000:1 may beused, but polymerization will be slow. It has been found that whencatalyst level is controlled within these parameters, catalytic activityis sufficient to polymerize the lactide while sufficiently low to enablemelt-processing without adverse effect when coupled with low residualmonomer levels and low water concentration as described above inpolymers of number average molecular weight between 10,000 to about300,000. It is believed in most applications the addition of astabilizing agent may be unnecessary if catalyst level is optimized.

It also has been found that catalyst concentration may be reducedsubsequent to polymerization by precipitation from a solvent. Thisproduces a resin with reduced catalyst concentration. In an alternativeembodiment, the catalyst means for catalyzing the polymerization oflactide to form the polylactide polymer chains which was incorporatedinto the melt-stable lactide polymer composition during polymerizationis deactivated by including in the melt-stable lactide polymercomposition a catalyst deactivating agent in amounts sufficient toreduce catalytic depolymerization of the polylactide polymer. chains.Such catalyst-deactivating agents include hindered, alkyl, aryl andphenolic hydrazides, amides of aliphatic and aromatic mono- anddicarboxylic acids, cyclic amides, hydrazones and bishydrazones ofaliphatic and aromatic aldehydes, hydrazides of aliphatic and aromaticmono- and dicarboxylic acids, bis-acylated hydrazine derivatives, andheterocyclic compounds. A preferred metal deactivator is Irganox® MD1024from Ciba-Geigy.

In an alternative embodiment, the catalyst concentration is reduced tonear zero by utilizing a solid-supported catalyst to polymerize lactide.It is believed catalysts which may be utilized include supported metalcatalysts, solid acid catalysts, acid clays, alumina silicates, alumina,silica and mixtures thereof.

A preferred melt-stable lactide polymer composition is the reactionproduct of polymerization of lactide at a temperature greater than about160° C. Applicants have found that polymerization at higher temperaturesresults in a characteristically different polymer which is believed tohave higher melt stability due to increased transesterification duringpolymerization.

If the lactide polymer composition is used as a coating, as detailed inpending U.S. application Ser. No. 08/034,099 which was filed on Mar. 22,1993 and is a continuation in part of U.S. Pat. No. 07/955,690, thedisclosure of which is hereby incorporated by reference, a plasticizermay be included in the polymer formulation in order to improve thecoating quality of the polymer. More particularly, plasticizers reducethe glass transition temperature of poly(lactide), which aides inprocessing and coating the polymer at lower temperatures and may improveflexibility and reduce cracking tendencies of the coated product.

Selection of a plasticizing agent requires screening of many potentialcompounds and consideration of several criteria. For use in abiodegradable coating the preferred plasticizer is to be biodegradable,non-toxic, compatible with the resin and relatively nonvolatile.

Plasticizers in the general classes of alkyl or aliphatic esters, ether,and multi-functional esters and/or ethers are preferred. These includealkyl phosphate esters, dialkylether diesters, tricarboxylic esters,epoxidized oils and esters, polyesters, polyglycol diesters, alkylalkylether diesters, aliphatic diesters, alkylether monoesters, citrateesters, dicarboxylic esters, vegetable oils and their derivatives, andesters of glycerine. Most preferred plasticizers are tricarboxylicesters, citrate esters, esters of glycerine and dicarboxylic esters.These esters are anticipated to be biodegradable. Plasticizerscontaining aromatic functionality or halogens are not preferred becauseof their possible negative impact on the environment.

For example, appropriate non-toxic character iS exhibited by triethylcitrate, acetyltriethyl citrate, tri-n-butyl citrate, acetyltri-n-butylcitrate, acetyltri-n-hexyl citrate, n-butyryltri-n-hexyl citrate anddioctyl adipate.

Appropriate compatibility is exhibited by acetyltri-n-butyl citrate andtriethyl citrate. Other compatible plasticizers include any plasticizersor combination of plasticizers which can be blended with poly(lactide)and are either miscible with poly(lactide) or which form a mechanicallystable blend. Corn oil and mineral oil were found to be incompatiblewhen used alone with poly(lactide) because of phase separation (notmechanically stable) and migration of the plasticizer.

Volatility is determined by the vapor pressure of the plasticizer. Anappropriate plasticizer must be sufficiently non-volatile such that theplasticizer stays substantially in the resin formulation throughout theprocess needed to produce the coating. Excessive volatility can lead tofouling of process equipment, which is observed when producing films bymelt processing poly(lactide) with a high lactide content. Preferredplasticizers should have a vapor pressure of less than about 10 mm Hg at170° C., more preferred plasticizers should have a vapor pressure ofless than 10 mm Hg at 200° C. Lactide, which is not a preferredplasticizer, has a vapor pressure of about 40 mm Hg at 170° C.

In a preferred composition for some applications, fillers may be usefulto prevent blocking or sticking of the coated product during storage andtransport. Inorganic fillers include clays and minerals, either surfacemodified or not. Examples include talc, silica, mica, kaolin, titaniumdioxide, and wollastonite. Preferred inorganic fillers areenvironmentally stable and non-toxic. Some fillers, such as talc, havebeen found to act as nucleating agents, increasing the rate ofcrystallization.

Organic fillers include a variety of forest and agricultural products,either with or without modification. Examples include cellulose, wheat,starch, modified starch, chitin, chitosan, keratin, cellulosic materialsderived from agricultural products, gluten, nut shell flour, wood flour,corn cob flour, and guar gum. Fillers may be used either alone or asmixtures of two or more fillers.

Surface treatments may also be used to reduce blocking. Such treatmentsinclude dusting the surface with materials which reduce the surfacecontact between the poly(lactide) based coating and the adjacentsurface. Examples of materials which may be used in surface treatmentsinclude talc, silica, corn starch, corn meal, latex spheres or otherparticulates. Celite® Super Floss commercially available from CeliteCorp. has been found to be effective.

For certain applications, it is desirable for the coating to have goodslip properties. Lubricating solids such as fluoropolymer powders orgraphite are sometimes incorporated into materials to increase slipproperties. The fatty acid esters or hydrocarbon waxes commonly used aslubricants for the melt state, are gradually exuded, if used in veryhigh concentrations, thus yielding to permanent lubricating effects.Certain additives migrate so strongly to the surface, even duringcooling, that a uniform invisibly thin coating is formed. Thus, theseslip agents may be important in the production of coatings which areused in automatic packaging machines.

Antistatic agents may be employed in the present invention. Antistaticagents are surfactants which can be subdivided into cationic, anionic,and nonionic agents.

Pigments or color agents may also be added as necessary. Examplesinclude titanium dioxide, clays, calcium carbonate, talc, mica, silica,silicates, iron oxides and hydroxides, carbon black and magnesium oxide.

The resulting polylactide should also exhibit reduced neck-in whencompared with linear non-functionalized polylactide of a comparablemolecular weight. In order to determine whether the neck-in of thepolylactide is reduced, any method well known in the art can be used.The following method is useable. A polylactide polymer film is extrudedunder the following conditions. An extruder with a suitable film die,for example, a one-inch extruder with a six-inch film die and chill rollstack, is used. The extruder is set at conditions suitable to produce anextrusion cast film using a linear polymer with a number averagemolecular weight comparable to the test polymer. The number averagemolecular weight of the linear polylactide should be within 20% of theless linear polylactide test sample. Typical die temperatures forpolylactide are 160° C. to about 180° C. The extruder speed and take uproll speed are adjusted to produce a film of about 0.5 to about 3.0 milthickness. The neck-in is determined as the die width minus the finishedfilm width. The test polymer should be run at the same conditions as thelinear control polymer, and the test sample's neck-in should bedetermined in the same manner. The neck-in ratio is the neck-in of thetest sample (modified) polymer divided by the neck-in for the linearcontrol polymer. Improvement of significance has occurred if a neck-inratio of less than about 0.8 is obtained. Preferred improvement hasresulted if the neck-in ratio is less than about 0.4.

Melt-Stable Lactide Polymer Process

The process for the manufacture of a melt-stable lactide polymercomprises the steps of first providing a purified lactide mixture, suchas that produced in the process disclosed by Gruber et al. in U.S. Pat.Nos. 5,247,059 and 5,244,073, although the source of lactide is notcritical to the process of the present invention.

The lactide mixture is polymerized to form a lactide polymer orpolylactide with some residual unreacted monomer in the presence of acatalyst means for catalyzing the polymerization of lactide to formpolylactide. Catalysts suitable for such polymerization have been listedpreviously. The concentration of catalysts utilized may be optimized asdiscussed previously.

In a preferred embodiment, a stabilizing agent as disclosed above, whichmay be an antioxidant and/or a water scavenger is added to the lactidepolymer. It is recognized that such stabilizing agents may be addedsimultaneously with or prior to the polymerization of the lactide toform the lactide polymer. The stabilizing agent may also be addedsubsequent to polymerization.

The lactide polymer is then devolatilized to remove unreacted monomerwhich may also be a by-product of decomposition reactions or theequilibrium-driven depolymerization of polylactide. Any residual waterwhich may be present in the polymer would also be removed duringdevolatilization, although it is recognized that a separate drying stepmay be utilized to reduce the water concentration to less than about1,000 parts-per-million. The devolatilization of the lactide polymer maytake place in any known devolatilization process. The key to selectionof a process is operation at an elevated temperature and usually underconditions of vacuum to allow separation of the volatile components fromthe polymer. Such processes include a stirred tank devolatilization or amelt-extrusion process which includes a devolatilization chamber and thelike.

In a preferred process for manufacture of a melt-stable lactide polymercomposition, the process also includes the step of adding a molecularweight control agent to the lactide prior to catalyzing thepolymerization of the lactide. Molecular weight control agents includeactive hydrogen-bearing compounds, such as lactic acid, esters of lacticacid, alcohols, amines, glycols, diols and triols which function aschain-initiating agents. Such molecular weight control agents are addedin sufficient quantity to control the number average molecular weight ofthe polylactide to between about 10,000 and about 300,000.

Next referring to FIG. 1 which illustrates a preferred process forproducing a melt-stable lactide polymer composition. A mixture oflactides enters a mixing vessel (3) through a pipeline (1). A catalystfor polymerizing lactide is also added through a pipeline (13). Withinmixing vessel (3) a stabilizing agent may be added through a pipeline(2). A water scavenger may also be added through the pipeline (2). Thestabilized lactide mixture is fed through a pipeline (4) to apolymerization process (5) which may be conducted at temperaturesgreater than 160° C. The polymerized lactide or lactide polymer leavesthe polymerization process through a pipeline (6). The stream is fed toa second mixing vessel (8) within which a stabilizing agent and/orcatalyst deactivating agent may be added through a pipeline (7). Thestabilized lactide polymer composition is then fed to a devolatilizationprocess (10) through a pipeline (9). Volatile components leave thedevolatilization process through a pipeline (11) and the devolatilizedlactide polymer composition leaves the devolatilization process (10) ina pipeline (12). The devolatilized lactide composition is fed to aresin-finishing process (14). Within the resin-finishing process thepolymer is solidified and processed to form a pelletized or granularresin or bead. Applicants recognize the polymer may be solidified andprocessed to form resin or bead first, followed by devolatilization. Theresin is then fed to a drying process (16) by conveyance means (15).Within the drying process (16) moisture is removed as a vapor throughpipeline (17). The dried lactide polymer resin leaves the drying process(16) by a conveyance means (18) and is fed to a melt-processingapparatus (19). Within the melt-processing apparatus (19) the resin isconverted to a useful article as disclosed above. The useful articleleaves the melt-processing apparatus (19) through a conveyance means(20). The process illustrated in FIG. 1 can be readily conducted as acontinuous process.

The various agents (for example, radical initiators, non-initiatingreactants or initiating reactants) useable to provide the improvedpolymers as discussed herein may be added at various points in theprocess. For example, at mixing vessel 3, in the polymerization reactor,at vessel 8, in devolatilize 10, or in subsequent processing steps.

One example of a useful article, is a coated paper article. A typicalmethod of coating paper, as disclosed in U.S. application Ser. No.08/034,099, which was filed on Mar. 22, 1993 and which is herebyincorporated by reference, is by extruding a melt through a die onto amoving substrate. After the coating process, the paper may be calendaredto improve surface properties such as smoothness and gloss. In thecalendaring process, the coated paper passes through alternating hardand soft rolls which reform the surface, often producing a gloss whilesmoothing or leveling surface face contours.

EXAMPLES

Examples 1 through 10 and 16-18 disclose methods and compositionsutilizing a non-initiating lactide reactant as discussed previously withrespect to configurations (5)-(8). Examples 11-15 disclose methods andcompositions utilizing peroxides and free radical reaction, aspreviously discussed. In the examples, Mn=number average molecularweight as determined by gel permeation chromatography (GPC); Mw=weightaverage molecular weight by GPC. Mz is the sum of the product of thenumber of molecules of a molecular weight times the cube of thatmolecular weight, divided by the sum of the number of molecules of amolecular weight times the square of that molecular weight.

Example 1 Copolymerization of Lactide with Epoxidized Soybean Oil andEpoxidized Tall Oil

Epoxidized soybean oil (FLEXOL® EPO, commercially available from UnionCarbide) and epoxidized tall oil (FLEXOL® EP8, commercially availablefrom Union Carbide) were separately copolymerized with lactide. Aphosphite based process stabilizer (Weston TNPP, commercially availablefrom General Electric) was added to the lactide at 0.4 weight percent.Catalyst (2-Ethylhexanoic acid, tin(II) salt from Aldrich Co.,Milwaukee, Wiss.) in a tetrahydrofuran carrier was added in a molarratio 1 part catalyst/10,000 parts lactide. Mixtures of the moltenlactide, epoxidized oil, stabilizer, and catalyst were sealed in vialsand polymerized at 180° C. for 2.5 hours. The samples were thendissolved in chloroform and analyzed by gel permeation chromatographyusing a refractive index detector and Ultrastyragel® IR column fromWaters Chromatography to determine weight average and number averagemolecular weights for the resulting copolymer resins. The systemtemperature was 35° C. and the GPC column was calibrated againstpoly(styrene) standards. The results of these tests appear in Table 1.

                  TABLE 1                                                         ______________________________________                                                        Weight Average                                                Sample          Mol. Weight  % Conversion                                     ______________________________________                                        control poly(lactide)                                                                         240,000      71                                               copolymerized with 1.0 wt                                                                     400,000      96                                               % epoxidized soybean oil                                                      copolymerized with 1.5 wt                                                                     178,000      96                                               % epoxidized tall oil                                                         ______________________________________                                    

The results for the epoxidized soybean oil show a significant increasein the weight average molecular weight, indicative of a coupling orcrosslinking mechanism during the copolymerization. This is attributedto the multiple oxirane functionality contained in most of theepoxidized soybean oil molecules (an average of about 4.6 oxiraneoxygens/molecule). The epoxidized tall oil copolymer does not show anincrease in weight average molecular weight, presumably because each ofthe tall oil molecules contain an average of only about 1 oxirane group.The results for both the epoxidized tall oil and the epoxidized soybeanoil show an increase in reaction rate for the copolymerization,achieving 96% conversion of the monomers, while the control reactiononly exhibited 71% conversion.

Example 2 Examples of Epoxidized Linseed Oil as a Copolymerizing Agent

A copolymerized poly(lactide) was produced by adding epoxidized linseedoil to a continuous pilot plant polymerization of lactide in the samemanner described in Example 1. This was accomplished by adding asolution of TNPP and epoxidized linseed oil (FLEXOL® Plasticizer LOEfrom Union Carbide), in a ratio of 1:2 by weight, at a rate of 10 gm/hrto the continuous polymerization such that the weight ratio ofepoxidized oil to lactide was 0.55. Lactic acid was processed intolactide in a continuous pilot scale reactor, purified by distillation,and fed to a continuous polymerization reactor system. Thepolymerization system consisted of a 1-gallon and a 5-gallon reactor inseries. The reactors are continuous feed, stirred tank reactors. Thelactide feed rate was 1.1 kg/hr, the catalyst, tin (II) bis(2-ethylhexanoate) (T-9 from Atochem) was added at a rate of 0.03 weightpercent. A phosphite process stabilizer (Weston TNPP® from GeneralElectric) was added at a rate of 0.3 weight percent. Reactortemperatures were 190° C. to 200° C. The resulting polymer pellets werebagged every eight hours and labelled as samples I-VII. The pellets weredried and collected for GPC analysis. Total run time was 52 hoursgenerating 60 kilograms material.

GPC results after drying

                  TABLE 2                                                         ______________________________________                                        Example    Time      Mn        Mw    PDI                                      ______________________________________                                        start      zero      89000     220000                                                                              2.5                                      I          0-8 hours 79000     307000                                                                              2.9                                      II          8-16 hours                                                                             50000     296000                                                                              5.0                                      III        16-24 hours                                                                             72200     323000                                                                              4.4                                      IV         24-32 hours                                                                             80900     339000                                                                              4.2                                      V          32-40 hours                                                                             81500     316000                                                                              3.9                                      VI         40-48 hours                                                                             76200     303000                                                                              4.0                                      VII        48-52 hours                                                                             81600     319000                                                                              4.0                                      ______________________________________                                    

The resulting material was then subjected to a devolatilization processto remove the residual amount of unreacted monomer lactide. Afterdevolatilization, samples III-VII were combined and used in furthertesting. Molecular weights of the combined fractions afterdevolatilization were: Mn-75,000 Mw-325000 PDI-4.3 and a residuallactide level of less than 0.5 percent as recorded by a GPC.

Example 3 Example of Vial Polymerizations with Epoxidized Oil, ShowingEffect on Rate of Polymerization

Tin(II) bis(2-ethylhexanoate) commercially available as 2-ethylhexanoicacid, tin(II) salt from Aldrich Chemical Company, and epoxidized linseedoil (FLEXOL® Plasticizer LOE from Union Carbide) were placed into avial. A molten mixture of 90% L-lactide and 10% D,L-lactide, with 0.4%by weight of a stabilizer (Weston TNPP), was then added to the vial. Anidentical set was made up without the epoxidized oil. In each case thefinal catalyst concentration was 1 part catalyst per 5000 parts lactideand the epoxidized oil was 1% by weight of the final reaction mixture.The solutions were sealed and placed in an oil bath at 180° C. Sampleswere pulled over time and analyzed by GPC for molecular weight andextent of lactide conversion.

The experiment was repeated, except that the catalyst and the epoxidizedoil were added to the molten lactide before it was placed in therespective vials.

The results of both experiments are shown in Tables 3 and 4respectively. The epoxidized oil resulted in an increase in thepolymerization reaction rate in each study. The weight average molecularweight and PDI (polydispersion index) are also higher.

                  TABLE 3                                                         ______________________________________                                        Sample  Time (min.)                                                                             % Conversion                                                                              Mn    Mw    PDI                                 ______________________________________                                        Control 15        10           6800  7800 1.12                                        30        16          39100 40600 1.04                                        45        48          30400 40100 1.32                                        60        73          48900 77800 1.59                                        90        78          54000 86200 1.60                                With 1% 15        12           7800  8800 1.12                                epoxidized                                                                            30        69          57100 115000                                                                              2.01                                oil     45        74          50500 112000                                                                              2.22                                        60        80          67300 123000                                                                              1.82                                        90        93          78400 176000                                                                              2.25                                ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Time (min.)   % Conversion                                                                              Mn      Mw    PDI                                   ______________________________________                                        Control 15         0          --    --    --                                          30         8           5400  5700 1.05                                        45        18          14500 16500 1.14                                        60        28          26400 29000 1.10                                        90        45          26900 29000 1.15                                With 1% 15        11           7500  8800 1.17                                epoxidized                                                                            30        32          24700 29700 1.22                                oil     45        57          31300 44000 1.40                                        60        69          50300 71000 1.41                                        90        84          53500 96400 1.80                                ______________________________________                                    

Example 4 Cast Film at Typical Extrusion Temperatures

Films of a control polymer and a copolymer of the present invention wereextruded. The conditions and the results follow:

Extruder

Equipment: Killion 1" extruder 30/1 L/D rate with a 6" cast sheetdisplaced about 1/2 inch from a three stack chill roll. The followingwere the temperatures (° F):

    ______________________________________                                                                                     Chill                            Zone 1                                                                              Zone 2  Zone 3  Zone 4                                                                              Adapter                                                                              Die  Melt Roll                             ______________________________________                                        300   330     350     350   335    330  340  100                              ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Cast film results: Base PLA (Mn = 70,000; Mw = 215,000)                             Screw          Take                                                     Power Speed   Press  Off   Thickness                                                                             Width  Neck-in                             (amps)                                                                              (rpm)   (psi)  Setting                                                                             (mils)  (inches)                                                                             (inches)                            ______________________________________                                        12.5  40      3840   2.0   17.0    5.125  0.875                               12.5  40      3840   4.0   8.0     4.625  1.375                               12.5  40      3840   6.0   5.5     4.375  1.625                               12.5  40      3840   8.0   4.0     4.250  1.75                                12.5  40      3840   10.0  2.5     4.0    2.0                                 12.0  30      3610   10.0  1.5     4.0    2.0                                 11.5  20      3380   10.0  1.0     3.75   2.25                                11.5  10      2850   10.0  0.7     3.75   2.25                                ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        PLA w/epoxidized linseed oil (Mn = 75,000; Mw = 325,000)                            Screw          Take                                                     Power Speed   Press  Off   Thickness                                                                             Width  Neck-in                             (amps)                                                                              (rpm)   (psi)  Setting                                                                             (mils)  (inches)                                                                             (inches)                            ______________________________________                                        5.5   40      1950   2.0   12.0    5.0    1.0                                 5.0   40      1950   4.0   8.5     5.0    1.0                                 5.0   40      1950   6.0   5.5     4.75   1.25                                5.0   40      1950   8.0   4.0     4.75   1.25                                5.0   40      1950   10.0  3.5     4.75   1.25                                5.0   30      1650   10.0  2.0     4.75   1.25                                5.0   20      1250   10.0  1.0     4.75   1.25                                4.5   10       880   10.0  0.5     4.75   1.25                                ______________________________________                                    

The results show that poly(lactide) co-polymerized with epoxidizedlinseed oil processes at lower power consumption and pressure, andgenerates a polymer with reduced neck-in.

Example 5 Cast Film at Reduced Extrusion Temperatures

Separate films made from a poly(lactide) control polymer and from thecopolymer of the present invention described in Example 2 were extrudedunder various conditions. The resulting films were then evaluated usingstandard measuring techniques. The extruding conditions and the datagathered from this evaluation are set forth below:

    ______________________________________                                        Extruder Temperatures (°F.) of:                                                                                     Chill                            Zone 1                                                                              Zone 2  Zone 3  Zone 4                                                                              Adapter                                                                              Die  Melt Roll                             ______________________________________                                        285   295     305     305   305    305  305  100                              ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        Cast film results: PLA w/epoxidized linseed oil                                     Screw          Take                                                     Power Speed   Press  Off   Thickness                                                                             Width  Neck-in                             (amps)                                                                              (rpm)   (psi)  Setting                                                                             (mils)  (inches)                                                                             (inches)                            ______________________________________                                        10.5  40      3470   2.0   10.0    5.125  0.875                               10.0  40      3470   4.0   6.0     5.125  0.875                               10.0  40      3470   6.0   4.0     5.125  0.875                               10.0  40      3470   8.0   3.5     5.0    1.0                                 10.0  10      3470   10.0  2.5     5.0    1.0                                 7.5   30      3250   10.0  1.5     5.0    1.0                                 6.0   20      2720   10.0  0.7     5.0    1.0                                 6.0   10      2000   10.0  0.5     5.125  0.875                               2.5   4.5     1450   10.0  0.25    5.25   0.75                                2.5   1.0      920   10.0  0.1     5.25   0.75                                ______________________________________                                    

Under similar extrusion temperatures, the control poly(lactide) couldnot run because the power consumption exceeded maximum levels (>15amps). The results show that poly(lactide) polymerized with epoxidizedlinseed oil has the benefit of processing at lower temperatures andgenerates a polymer with increased melt strength, less neck-in and afilm of lower thickness.

Example 6 Blown Film of Base Poly(lactide) with Epoxidized Linseed Oil

A copolymer of lactide with epoxidized linseed oil was prepared in themanner described in Example 2 and was blown into a 8 inch width film atthickness from 3.0 to 0.5 mils. The blown film line consisted of aKillion tower connected to a Killion 1" extruder 30:1 L/D ratio equippedwith a 2.25 inch blown film die. Distance from the die to the towers niproll was 2.5 feet.

                  TABLE 8                                                         ______________________________________                                        Extruder Temperatures (°F.):                                                                                        Chill                            Zone 1                                                                              Zone 2  Zone 3  Zone 4                                                                              Adapter                                                                              Die  Melt Roll                             ______________________________________                                        300   320     330     325   310    310  310  320                              ______________________________________                                    

Operation of the blown film line was very smooth.

Example 7 Use of Hydroxyl Initiators and Effect on Molecular Weight

L-lactide was melted under nitrogen and catalyst [tin (II) bis2-ethylhexanoate, 1:5000 molar ratio of tin to lactide] was added.Initiator was added at the rate of 1:500 molar basis, initiator tolactide. The samples were polymerized at 80° C. for 2 hours. Sampleswere then ground and devolatilized at 125° C. and 10 mmHg pressureovernight. Samples were reground, dissolved in chloroform, and analyzedby gel permeation chromatography (GPC) against polystyrene standards.The results are shown below:

    ______________________________________                                        Initiator    Mn          Mw       PDI                                         ______________________________________                                        Dodecanol    54,800      113,000  2.06                                        2-EHMPD      55,400      95,000   1.72                                        Dipentaerythritol                                                                          56,400      93,600   1.66                                        ______________________________________                                    

2-EHMPD is 2-ethyl-2-(hydroxy methyl)-1,3-propane diol. The numberaverage molecular weights are consistent with the expected values foradding hydroxyl initiators. The low PDI (PDI<2) are consistent with themost probable distribution for multi-functional initiators. The PDI arelower than the PDI of about 2.0 which is typically seen for vialpolymerization of lactide.

Example 8 Lactide/Epoxidized Soybean Oil Copolymerization

Lactide was copolymerized with epoxidized soybean oil in a continuouspilot line. The feed contained 0.25 weight percent epoxidized soybeanoil [Paraplex G-62; C. P. Hall], 0.1 weight % PNPG process stabilizer[Weston], and 0.03 weight percent catalyst (tin II) bis(2-ethylhexanoate). Two back-mixed reactors in series (1 gallon and 5 gallon)were used. The reaction temperature was about 215° C., and the reactorswere about 75% full.

The copolymer had a number average molecular weight of about 70,000 anda weight average molecular weight of about 210,000, giving a PDI ofabout 3.0. Under similar conditions, but without the epoxidized oil, thepilot line produced poly(lactide) with a PDI of 2.1-2.5 and comparablenumber average molecular weight.

Example 9 Neck-in on Cast Film Using Epoxidized Soybean Oil Modified PLA

A performance comparison for extruding a cast sheet was made usingnormal, linear poly(lactide) and the less linear poly(lactide)copolymerized with epoxidized soybean oil) from Example 8. The test wasconducted using a 1" Killion extruder with 30/1 L/D connected to a 6"cast sheet die. The die was approximately 2 inch from a three roll chillstack. The extruder die temperature was 345° F. and the chill roll was100° F. The following table presents the measured power usage, diepressure, and film neck-in (die width--minimum sheet width) for basepoly(lactide) and the modified polymer. The take-off setting was heldconstant.

    __________________________________________________________________________    Base Poly(lactide)                                                            Screw                                                                             Approx            Modified Poly(lactide)                                  Speed                                                                             Thick                                                                              Power                                                                             Press                                                                             Neck-in                                                                            Power                                                                             Press                                                                             Neck-in                                                                            Neck-in                                    (rpm)                                                                             (mil)                                                                              (amps)                                                                            (psi)                                                                             (inches)                                                                           (amps)                                                                            (psi)                                                                             (inches)                                                                           ratio                                      __________________________________________________________________________    40  3    15  1260                                                                              1.75 8   785 0.7  0.40                                       30  2    12.5                                                                              1090                                                                              1.75 8   650 0.9  0.51                                       20  1.5  12.5                                                                              860 1.75 7.5 510 1.0  0.57                                       10  0.8  10.5                                                                              560 2.0  6.0 300 1.0  0.50                                        5  0.5  7.0 280 3.25 6.0 190 1.25 0.38                                       __________________________________________________________________________

The modified polymer shows benefits, at all screw speeds, of reducedpower consumption, reduced die pressure, and reduced neck-in.

Example 10 Curtain Coating with Epoxidized Soybean Oil Modified PLA

A comparison of linear poly(lactide) and modified poly(lactide)copolymer from Example 8 was made on an extrusion curtain coating line.The linear poly(lactide) had a number average molecular weight of 95,000with a PDI of 2.34, and the modified polymer had a number averagemolecular weight of 70,000 and PDI of 3.08.

The extrusion curtain coating line consisted of a 1.5" extruder with a24:1 L/D, connected to a vertical 13" coat hanger die. The extruder wasrun with a die temperature of 425° F. The polymer was coated onto 15pound basis weight kraft paper at a speed of 150 feet per minute. Thedie was held 3" above the substrate. The polymer through-put was variedusing the screw speed of the extruder to produce coatings of variousthicknesses. The table below shows the power consumption, coatingthickness, and amount of neck-in (die width--minimum coating width) atvarious screw speeds.

    ______________________________________                                        Linear Poly(lactide)                                                          Screw                   Modified Poly(lactide)                                Speed Power    Thick   Neck-in                                                                              Power Thick Neck-in                             (rpm) (amps    (Mil)   (inch) (amps)                                                                              (mil) (inch)                              ______________________________________                                        90    15       1.5     4      10    1.5   1.0                                 60    12       1.0     5      5     1.0   1.5                                 45    12       0.6     5      4     0.5   2.0                                 30    10       0.4     5      4     0.4   2.0                                 ______________________________________                                    

The linear polymer exhibited a very uneven coating action, with the edgeof the coating weaving in and out to make a coating of uneven width.Both materials showed excellent adhesion to the paper and producedcoatings free of tears or gels.

Example 11 Peroxide Treatment of Plasticized Poly(Lactide)

Poly(lactide) with 10.5 weight percent acetyl tri-n-butyl citrate as aplasticizer was blended with 0.25% and 0.5% dicumyl peroxide. Theperoxide was misted onto the pellets as a 50% solution in acetone,followed by vacuum drying at room temperature for 3 hours to remove theacetone. The pellets were then injection molded at 180 C. with a holdtime of 4.5 minutes. Molecular weights were determined by GPC. Gelcontent was determined as the residue remaining after dissolving at 1%in refluxing acetone for 3 hours and filtering. The table below showsthe change in molecular weight profile after treatment. The increase inhigh molecular weight components is consistent with bridging due toperoxide induced crosslinking.

    ______________________________________                                        Control         0.25% treated                                                                            0.50% treated                                      ______________________________________                                        Mn      64,000      87,000     82,000                                         Mw      170,000     326,000    456,000                                        Mz      376,000     1,162,000  1,184,000                                      PDI     2.65        3.73       5.49                                           % gel   0.0%        1.5%       2.1%                                           ______________________________________                                    

The 0.25% treated sample was slightly hazy, the 0.5% treated sample wasdull and hazy. Material properties of glass transition temperature,melting point, annealed percent crystallinity, break stress, modulus,and heat distortion temperature were unchanged.

Example 12 Peroxide Treatment/Neck-in on Cast Films

Poly(lactide) pellets were coated with 0.2 weight percent of eitherLupersol 101 or Lupersol TBEC (ELF Atochem) and processed in an extruderto make an extrusion cast film using a 6" die. The die temperature ofthe extruder was 335° F. with a residence time of about 4 minutes. Thetable below presents molecular weight distributions as determined by GPCand gel content as measured by acetone insolubles.

    ______________________________________                                        Base            TBEC Modified                                                                             101 Modified                                      ______________________________________                                        Mn    66,900        74,400      67,600                                        Mw    161,000       200,000     184,000                                       Mz    306,000       423,000     376,000                                       PDI   2.40          2.69        2.73                                          % gel 0.0%          1.0%        0.0%                                          ______________________________________                                    

All films were clear (non-hazy).

The neck-in was determined as the die width minus the film width.

    ______________________________________                                        Thick          Neck-in and neck-in ratio                                      (mil)  Base    TBEC       (ratio)  101   (ratio)                              ______________________________________                                        0.5    2.5     1.2        0.48     1.2   0.48                                 1.0    2.4     1.2        0.50     1.2   0.50                                 ______________________________________                                    

Example 13 Peroxide Treatment/Neck-in on Cast Films

A blend of plasticizer (acetyl tri-n-butyl citrate) and peroxide (ethyl3,3-bis-(t-butylperoxy)-butyrate) (commercially available is Luperco233XL from ELF Atochem was compounded with poly(lactide) and 4 weight %Celite Super Floss (Celite) diatomaceous earth using a Leistritz twinscrew extruder. The material was pelletized and dried, with molecularweights as shown below.

    ______________________________________                                        Sample % peroxide                                                                              % plasticizer                                                                             Mn    Mw    PDI                                  ______________________________________                                        1      0.0       0.0         77,000                                                                              165,000                                                                             2.13                                 2      0.10      20          86,500                                                                              197,000                                                                             2.28                                 3      0.25      15          81,800                                                                              219,000                                                                             2.68                                 4      0.50      20          72,300                                                                              261,000                                                                             3.61                                 5      1.00      15          61,400                                                                              243,000                                                                             3.96                                 6      1.00      20          71,800                                                                              275,000                                                                             3.83                                 ______________________________________                                    

The increase in high molecular weight components, as seen in the Mw andthe PDI, with increasing peroxide level is clearly evident.

Samples were tested for neck-in under extrusion cast film conditionsusing a 6" extrusion die. The neck-in is measured as the width of thedie (6") minus actual sheet width (inches). The following values wereobtained as a function of extruder screw speed.

    ______________________________________                                                Neck-in (inches)                                                                             Neck-in ratio                                          Screw Thick   Film   Film Film  Film Film Film  Film                          Speed (mil)   1      3    4     6    3    4     6                             ______________________________________                                        40    3.0     1.96   1.28 0.50  0.41 0.65 0.26  0.21                          30    2.0     2.06   1.15 0.50  0.34 0.56 0.24  0.17                          20    1.5     2.13   1.31 0.56  0.25 0.61 0.26  0.26                          10    0.8     2.35   1.19 0.38       0.51 0.16                                5     0.4     2.90   1.19 0.31       0.41 0.11                                ______________________________________                                    

Increasing peroxide clearly reduced neck-in at all screw speeds.

The following amps and die pressure were measured at various screwspeeds. Melt temperature for all tests was 165°-172° C.

Amps during sheet extrusion:

    ______________________________________                                        Screw Speed                                                                            Amps                                                                 (rpm)    Film 1   Film 3  Film 4 Film 5 Film 6                                ______________________________________                                        40       14.5     7       4.5    7      3.5                                   30       14       6.5     4      5.5    3                                     20       13       6       3.2    3.5    2.8                                   10       11       5       2.8    3                                            5        8.5      3       2                                                   ______________________________________                                    

Die pressure during sheet extrusion:

    ______________________________________                                        Screw Speed                                                                            Die Pressure (psi)                                                   (rpm)    Film 1   Film 3  Film 4 Film 5 Film 6                                ______________________________________                                        40       1150     800     720    920    730                                   30       970      680     640    770    600                                   20       770      560     530    630    480                                   10       495      400     380    490                                          5        310      300     270                                                 ______________________________________                                    

The large drop in amps and die pressure between film 1 and the others ispresumably due to the addition of plasticizer in the other formulations.To see the effect of peroxide, we compare films 3 and 5 (0.25% and 1%)peroxide at constant 15% plasticizer) and films 4 and 6 (0.50% and 1%peroxide at constant 20% plasticizer). The peroxide seems to have causeda slight decrease in amperage but uncertain (possible increase) effecton die pressure.

Example 14 Peroxide Treatment/Blown Film Results

Samples 3, 4, 5, and 6 from Example 13 were blown into 2 rail film usinga Killion extruder with a 2.25" blown film die and a Killion tower. Thematerials formed blown films with less difficulty than linearpoly(lactide). Film properties from tensile and trouser tear testresults are shown below. The tensile test is provided in ASTM D882 andthe trouser tear test is exemplified by ASTM D1938.

    ______________________________________                                               % elong. % elong.  tensile break                                                                          tear break                                 Sample at yield break     energy (in-lb)                                                                         energy (in-lb)                             ______________________________________                                        3      6.7      10        1.4      0.30                                       5      3.6      4         1.2      0.28                                       6      8.1      368       70.4     0.15                                       4      11.5     491       78.0     0.23                                       ______________________________________                                    

EXAMPLE 15 Effect of Peroxide Treatment on Shear Viscosity as Determinedby Capillary Rheometry

A series of polymers, with 15% plasticizer and various levels ofperoxide (Luperco 233XL), prepared in a manner similar to those inExample 13 were tested using a capillary viscometer at a temperature of175° C. The viscosity data are shown in the table below.

    ______________________________________                                                       Apparent Shear Viscosity (Pa-S)                                Material   Shear rate                                                                              500/sec  1000/sec                                                                             5000/sec                                 ______________________________________                                        0.1% peroxide        198      95     26                                       0.25% peroxide       258      118    33                                       1.0% peroxide        267      123    39                                       ______________________________________                                    

The data show that at increasing levels of peroxide the apparent shearviscosity increases. This is consistent with increased molecular weightdue to bridging.

Example 16 Intrinsic Viscosity vs. Molecular Weight Data

A series of linear non-functionalized poly(lactide) samples wereprepared using vial polymerizations with lactic acid added as amolecular weight control agent. These samples were dried anddevolatilized, then dissolved in chloroform for GPC molecular weightdetermination (relative to polystyrene standards) and for intrinsicviscosity (IV). Both the GPC and the intrinsic viscosity were carriedout at 35° C. The intrinsic viscosity measurements were made at three ormore concentration points and extrapolated to zero concentration,following standard procedure.

A branched poly(lactide) copolymer with epoxidized linseed oil, fromExample 2, was also tested in this manner.

The results are shown in FIG. 2, where ln(IV) is plotted vs ln(apparentweight average mol weight). (I.V. is measured in deciliters/gram.) Fortypical poly(lactide), with a PDI of about 2, all the points areexpected to fall on a single line, determined by the Mark-Houwinkconstants. A branched polymer, with sufficiently long arms, is expectedto have a smaller radius of gyration and exhibit a lower intrinsicviscosity at a given molecular weight. The figure shows intrinsicviscosity relative to apparent molecular weight, which in this case isequivalent to GPC retention time and therefore to hydrodynamic volume.It can be shown that a branched polymer, because of its smaller radiusof gyration, has a higher molecular weight and lower IV at a givenhydrodynamic volume. The point for the modified polymer is an example ofthis.

Each of the linear poly(lactides) falls within 0.07 units of the ln(IV)vs ln (apparent weight average mol weight) line. The modified polymer is0.5 units lower than predicted by that line. According to the testdescribed above, this is an example of a poly(lactide) with long chainbranching and thus increased molecular interaction.

Example 17 Comparison of Copolymerized Epoxidized Oil with Blending ofEpoxidized Oil

Polymer samples of base poly(lactide), base poly(lactide) compounded inan extruder with 0.2% and 0.5% epoxidized soybean oil (ESO), and acopolymer of poly(lactide) with about 0.3% epoxidized soybean oil weretested for apparent shear viscosity using a capillary viscometer.Molecular weight data, determined by gel permeation chromatography, areshown below.

    ______________________________________                                        Sample        Mn          Mw      PDI                                         ______________________________________                                        Base poly(lactide)                                                                          76,000      176,000 2.3                                         Base + 0.2% ESO                                                                             70,000      158,000 2.3                                         Base + 0.5% ESO                                                                             66,000      151,000 2.3                                         Copolymer     50,000      213,000 4.8                                         ______________________________________                                    

Results of the capillary viscosity testing at 175° C. are shown in FIGS.3 and 4. The copolymer is seen to have a dramatically lower apparentshear viscosity. The lower shear viscosity at higher weight averagemolecular weight is surprising, but is consistent with the reduced diepressure observed when processing the epoxidized oil copolymers inExamples 4 and 9.

Example 18 Screw Sticking Evaluation

An injection molding machine was set at 350° F., and the screw wasfilled with a test polymer. The test polymer was allowed to sit in thescrew for 2 minutes and then it was extruded at 500 psi. The actual rpmof the screw was monitored as the material was extruded. In the absenceof sticking, a maximum of 150 rpm was achieved. For base poly(lactide)(or linear non-functionalized polylactide) these conditions can resultin a screw which will not turn at all, due to sticking. The followingtable presents the results of testing the polymers from Example 17.

    ______________________________________                                        Sample            Screw speed (rpm)                                           ______________________________________                                        Base poly(lactide)                                                                              5-15                                                        Base + 0.2% ESO   2-15                                                        Base + 0.5% ESO   1-15                                                        Copolymer of lactide/ESO                                                                        135-152                                                     ______________________________________                                    

The table shows that, when processing the copolymer, the injectionmolder screw developed the full rpm--indicating less tendency to stick.This is a surprising and significant processing benefit of theepoxidized oil copolymer. This benefit is not obtained from a simplemixture of base poly(lactide) and epoxidized oil.

What is claimed is:
 1. A method of producing a polylactide polymercomposition; said method including the step of:(a) providing in thepolylactide polymer composition, polylactide polymer molecules whichhave been modified, relative to linear non-substituted polylactide, toprovide increased molecular interaction among polylactide backbonechains in the composition and a polydispersity index of at least 2.5. 2.A method according to claim 1 wherein said increased molecularinteraction is accomplished through providing a polymer compositionhaving at least one of the following, relative to linear non-substitutedpolylactide: an increased weight average molecular weight, increasedbranching and increased bridging.
 3. A method according to claim 2comprising producing a polymer having:(a) a mixture of polylactidepolymer chains having a number average molecular weight from about50,000 to about 300,000.
 4. A method according to claim 2 comprisingproducing a polymer having:(a) a mixture of polylactide polymer chainshaving a weight average molecular weight from about 100,000 to about1,200,000.
 5. A method according to claim 2 including:(a) generatingbridging between polylactide molecules by free radical reaction.
 6. Amethod according to claim 5 wherein said step of generating bridgingbetween polylactide molecules comprises providing a ratio of initiatorto polymer within the range of 0.01:1 to 10:1.
 7. A method according toclaim 1 wherein said step of providing modified polylactide polymermolecules comprises:(a) providing a polymer composition having ameasured natural log of intrinsic viscosity in deciliters per gram of atleast 0.1 below a measured natural log of intrinsic viscosity indeciliters per gram of a linear non-substituted polylactide of apparentcomparable weight average molecular weight as determined by gelpermeation chromatography.
 8. A method according to claim 1 wherein saidstep of providing modified polylactide polymer molecules comprises:(a)providing sufficient molecular interaction to produce a polymercomposition having reduced neck-in when processed, relative to a linearnon-substituted polylactide of comparable weight average molecularweight; said neck-in being reduced such that a neck-in ratio for saidpolymer composition is less than about 0.8.
 9. A method according toclaim 1 wherein said step of providing modified polylactide polymermolecules includes:(a) forming the polylactide molecules in a procedureincluding a reactant in addition to non-substituted lactic acid orlactide; said step of forming the polylactide molecules comprising:providing at least one of the following types of reactants:(i) anon-initiating lactide reactant; (ii) an initiating reactant; or, (iii)a combination reactant.
 10. A method according to claim 9 wherein saidreactant in addition to non-substituted lactic acid or lactide is areactant having more than two initiating groups therein; the more thantwo initiating groups each being selected from the group consisting of:hydroxy groups, amine groups, and mixtures thereof.
 11. A methodaccording to claim 9 wherein said reactant in addition non-substitutedlactic acid or lactide comprises a non-initiating lactide reactantcontaining at least two non-initiating groups each selected from thegroup consisting of epoxide groups, cyclic ester groups, andcombinations thereof.
 12. A method according to claim 9 wherein saidreactant other than non-substituted lactic acid or lactide comprises anon-initiating lactide reactant containing at least one carbon-carbondouble bond.
 13. A method according to claim 9 wherein said reactantother than non-substituted lactic acid or lactide includes a bulkyorganic polymer entangling group therein.
 14. A compositioncomprising:(a) a polylactide polymer having a weight average molecularweight of at least about 100,000 and a polydispersity of at least 2.5.15. A composition of claim 14 wherein said polylactide polymer has anumber average molecular weight of at least 50,000 and a weight averagemolecular weight of at least about 100,000 and not greater than about1,200,000.
 16. A composition according to claim 14 having a neck-inratio of less than about 0.8.
 17. A method of producing a polylactidepolymer composition having a number average molecular weight of at least50,000, from a lactide mixture which has not been recrystallized from asolvent; said method including a step of:(a) providing in thepolylactide polymer composition, polylactide polymer molecules whichhave been modified, relative to linear non-substituted polylactide, toprovide increased molecular interaction among polylactide backbonechains in the composition.
 18. A method according to claim 17including:(a) forming polylactide molecules in the composition in aprocedure including a reactant in addition to non-substituted lacticacid or lactide; said reactant in addition to non-substituted lacticacid or lactide comprising a non-initiating lactide reactant containingat least two non-initiating groups each selected from the groupconsisting of: epoxide groups; isocyanate groups; cyclic ester groups;and, combinations thereof.
 19. A method of producing an improvedpolylactide polymer composition including a step of:(a) formingpolylactide molecules in the composition in a procedure including areactant in addition to non-substituted lactic acid or lactide; saidreactant in addition to non-substituted lactic acid or lactidecomprising a non-initiating lactide reactant containing at least twonon-initiating groups each selected from epoxide groups.
 20. A method ofproducing a polylactide polymer composition; said method including thestep of:(a) modifying polylactide polymer molecules by introducingsufficient bridging between the polylactide polymer molecules to provideincreased molecular interaction among polylactide backbone chains,relative to linear polylactide which is not bridged, until apolydispersity index of at least 2.5 is achieved for the polylactidepolymer composition.
 21. A method according to claim 20, wherein saidstep of modifying polylactide polymer molecules comprises reacting afree radical initiator with the polylactide polymer molecules.
 22. Amethod according to claim 20, wherein said step of modifying polylactidepolymer molecules comprises reacting compounds with --OH terminus groupson the polylactide polymer molecules.
 23. A method according to claim20, wherein said step of modifying polylactide polymer molecules isconducted until a polydispersity index of at least about 3 is achievedfor the polylactide polymer composition.
 24. A method of producing apolylactide polymer composition; said method including the steps of:(a)introducing free radical generating peroxide to cleave substituents frompolylactide polymer molecules to form polymer radical for bonding withother polymer radical until a polydispersity index of at least 2.5 isachieved for the polylactide polymer composition.